High density molecular memory device

ABSTRACT

This invention provides novel high density memory devices that are electrically addressable permitting effective reading and writing, that provide a high memory density (e.g., 10 15  bits/cm 3 ), that provide a high degree of fault tolerance, and that are amenable to efficient chemical synthesis and chip fabrication. The devices are intrinsically latchable, defect tolerant, and support destructive or non-destructive read cycles. In a preferred embodiment, the device comprises a fixed electrode electrically coupled to a storage medium having a multiplicity of different and distinguishable oxidation states wherein data is stored in said oxidation states by the addition or withdrawal of one or more electrons from said storage medium via the electrically coupled electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Ser. No. 10/053,814, filed onJan. 18, 2002 now U.S. Pat. No. 6,657,884, which is a Divisional of U.S.Ser. No. 09/346,228, filed on Jul. 1, 1999, now U.S. Pat. No. 6,381,169,both of which are incorporated herein by reference in their entirety forall purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with government support under Grant NumberN00014-99-1-0357 from the Office of Naval Research. The Government ofthe United States of America may have certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to memory devices. In particular this inventionprovides a nonvolatile electronic memory device capable of storinginformation in extremely high density.

BACKGROUND OF THE INVENTION

Basic functions of a computer include information processing andstorage. In typical computer systems, these arithmetic, logic, andmemory operations are performed by devices that are capable ofreversibly switching between two states often referred to as “0” and“1.” In most cases, such switching devices are fabricated fromsemiconducting devices that perform these various functions and arecapable of switching between two states at a very high speed usingminimum amounts of electrical energy. Thus, for example, transistors andtransistor variants perform the basic switching and storage functions incomputers.

Because of the huge data storage requirements of modern computers, anew, compact, low-cost, very high capacity, high speed memoryconfiguration is needed. To reach this objective, molecular electronicswitches, wires, microsensors for chemical analysis, and opto-electroniccomponents for use in optical computing have been pursued. The principaladvantages of using molecules in these applications are high componentdensity (upwards of 10¹⁸ bits per square centimeter), increased responsespeeds, and high energy efficiency.

A variety of approaches have been proposed for molecular-based memorydevices. While these approaches generally employ molecular architecturesthat can be switched between two different states, all of the approachesdescribed to date have intrinsic limitations making their uses incomputational devices difficult or impractical.

For example, such approaches to the production of molecular memorieshave involved photochromic dyes, electrochromic dyes, redox dyes, andmolecular machines. Each of these approaches, however, has intrinsiclimitations that ultimately render it unsuitable for use in molecularmemories. For example, photochromic dyes change conformation in responseto the absorption of light (e.g. cis-trans interconversion of an alkene,ring opening of a spiropyran, interconversion between excited-states inbacteriorhodopsin, etc.). Typically, the molecular structure of the dyeis interconverted between two states that have distinct spectralproperties.

Reading and writing data with such photochromic dyes requires use oflight, often in the visible region (400–700 nm). Light-mediated datastorage has intrinsic diffraction-limited size constraints. Moreover,most photochromic schemes are limited to scanning and interrogating dyesdeposited on a surface and are not amenable to 3-D data storage. Evenwith near-field optical approaches, which might allow reliableencoding/reading of data elements of 100×100 nm dimensions(Nieto-Vesperinas and Garcia, N., eds. (1996) Optics at the NanometerScale, NATO ASI Series E, Vol. 319, Kluwer Academic Publishers:Dordrecht) the inherent restricted dimensionality (2-D) limits datadensity to 10¹⁰ bits/cm². Strategies for 3-dimensional reading andwriting of photochromic systems have been proposed that rely ontwo-photon excitation of dyes to encode data, and one-photon excitationto read the data (Birge et al. (1994) Amer. Sci. 82: 349–355,Parthenopoulos and Rentzepis (1989) Science, 245: 843–845), but it isbelieved that no high-density memory cubes have reached prototype stagein spite of the passage of at least a decade since their initialproposition. In addition, it is noted that these dyes often exhibitrelatively slow switching times ranging from microsecond to milliseconddurations.

Electrochromic dyes have been developed that undergo a slight change inabsorption spectrum upon application of an applied electric field(Liptay (1969) Angew. Chem., Int. Ed. Engl. 8: 177–188). The dyes mustbe oriented in a fixed direction with respect to the applied field.Quite high fields (>10⁷ V/cm) must be applied to observe an alteredabsorption spectrum which can result in heat/power dissipation problems.In addition, the change in the absorption spectrum is typically quitesmall, which can present detection difficulties. The dyes revert to theinitial state when the applied field is turned off.

Redox dyes have been developed that undergo a change in absorptionspectrum upon chemical or electrochemical reduction (typically a2-electron, 2-proton reduction) (Otsuki et al. (1996) Chem. Lett.847–848). Such systems afford bistable states (e.g.,quinone/hydroquinone, azo/hydrazo). Redox dyes have only been examinedin solution studies, where they have been proposed for applications asswitches and sensors (de Silva et al. (1997) Chem. Rev. 97: 1515–1566).On a solid substrate, electrochemical reduction would need to beaccompanied by a source of protons. The latter requirement may bedifficult to achieve on a solid substrate. Furthermore, any opticalreading scheme would pose the same 2-D limitations as described forphotochromic dyes.

Yet another approach involves the design of molecular machines (Anell etal. (1992) J. Am. Chem. Soc. 114: 193–218). These elegant moleculararchitectures have moving parts that can be switched from one positionto another by chemical or photochemical means. The chemically inducedsystems have applications as sensors but are not practical for memorystorage, while the photochemically induced systems have the samefundamental limitations as photochromic dyes. Moreover, methods have notyet been developed for delineating the conformation/structure of themolecular machine that are practical in any device applications. ¹H NMRspectroscopy, for example, is clearly the method of choice forelucidating structure/conformation for molecules in solution, but istotally impractical for interrogating a molecular memory element. Noneof the current architectures for molecular machines has been designedfor assembly on a solid substrate, an essential requirement in a viabledevice.

In summary, photochromic dyes, electrochromic dyes, redox-sensitivedyes, and molecular machines all have fundamental limitations that haveprecluded their application as viable memory elements. These moleculararchitectures are typically limited by reading/writing constraints.Furthermore, even in cases where the effective molecular bistability isobtained, the requirement for photochemical reading restricts the devicearchitecture to a 2-dimensional thin film. The achievable memory densityof such a film is unlikely to exceed 10¹⁰ bits/cm². Such limitationsgreatly diminish the appeal of these devices as viable molecular memoryelements.

SUMMARY OF THE INVENTION

This invention provides novel high density memory devices that areelectrically addressable permitting effective reading and writing, thatprovide a high memory density (e.g., 10¹⁵ bits/cm³), that provide a highdegree of fault tolerance, and that are amenable to efficient chemicalsynthesis and chip fabrication. The devices are intrinsically latchable,defect tolerant, and support destructive or non-destructive read cycles.

In a preferred embodiment, this invention provides an apparatus forstoring data (e.g., a “storage cell”). The storage cell includes a fixedelectrode electrically coupled to a “storage medium” having amultiplicity of different and distinguishable oxidation states wheredata is stored in the (preferably non-neutral) oxidation states by theaddition or withdrawal of one or more electrons from said storage mediumvia the electrically coupled electrode. In preferred storage cells, thestorage medium stores data at a density of at least one bit, preferablyat a density of at least 2 bits, more preferably at a density of atleast 3 bits, and most preferably at a density of at least 5, 8, 16, 32,or 64 bits per molecule. Thus, preferred storage media have at least 2,8, 16, 32, 64, 128 or 256 different and distinguishable oxidationstates. In particularly preferred embodiments, the bits are all storedin non-neutral oxidation states. In a most preferred embodiment, thedifferent and distinguishable oxidation states of the storage medium canbe set by a voltage difference no greater than about 5 volts, morepreferably no greater than about 2 volts, and most preferably no greaterthan about 1 volt.

The storage medium is electrically coupled to the electrode(s) by any ofa number of convenient methods including, but not limited to, covalentlinkage (direct or through a linker), ionic linkage, non-ionic“bonding”, simple juxtaposition/apposition of the storage medium to theelectrode(s), or simple proximity to the electrode(s) such that electrontunneling between the medium and the electrode(s) can occur. The storagemedium can contain or be juxtaposed to or layered with one or moredielectric material(s). Preferred dielectric materials are imbedded withcounterions (e.g. Nafion). The storage cells of this invention are fullyamenable to encapsulation (or other packaging) and can be provided in anumber of forms including, but not limited to, an integrated circuit oras a component of an integrated circuit, a non-encapsulated “chip”, etc.In some embodiments, the storage medium is electronically coupled to asecond electrode that is a reference electrode. In certain preferredembodiments, the storage medium is present in a single plane in thedevice. The apparatus of this invention can include the storage mediumpresent at a multiplicity of storage locations, and in certainconfigurations, each storage location and associated electrode(s) formsa separate storage cell. The storage present on a single plane in thedevice or on multiple planes and said storage locations are present onmultiple planes of said device. Virtually any number (e.g., 16, 32, 64,128, 512, 1024, 4096, etc.) of storage locations and/or storage cellscan be provided in the device. Each storage location can be addressed bya single electrode or by two or more electrodes. In other embodiments, asingle electrode can address multiple storage locations and/or multiplestorage cells.

In preferred embodiments, one or more of the electrode(s) is connectedto a voltage source (e.g. output of an integrated circuit, power supply,potentiostat, microprocessor (CPU), etc.) that can provide avoltage/signal for writing, reading, or refreshing the storage cell(s).One or more of the electrode(s) is preferably connected to a device(e.g., a voltammetric device, an amperometric device, a potentiometricdevice, etc.) to read the oxidation state of said storage medium. Inparticularly preferred embodiments, the device is an impedancespectrometer or a sinusoidal voltammeter. Various signal processingmethods can be provided to facilitate readout in the time domain or inthe frequency domain. Thus, in some embodiments, the readout device(s)provide a Fourier transform (or other frequency analysis) of the outputsignal from said electrode. In certain preferred embodiments, the devicerefreshes the oxidation state of said storage medium after reading saidoxidation state.

A wide variety of molecules can be used as storage molecules and hencecomprise the storage medium. Preferred molecules include, but are notlimited to a porphyrinic macrocycle, a metallocene, a linear polyene, acyclic polyene, a heteroatom-substituted linear polyene, aheteroatom-substituted cyclic polyene, a tetrathiafulvalene, atetraselenafulvalene, a metal coordination complex, a buckyball, atriarylamine, a 1,4-phenylenediamine, a xanthene, a flavin, a phenazine,a phenothiazine, an acridine, a quinoline, a 2,2′-bipyridyl, a4,4′-bipyridyl, a tetrathiotetracene, and a peri-bridged naphthalenedichalcogenide. Even more preferred molecules include a porphyrin, anexpanded porphyrin, a contracted porphyrin, a ferrocene, a linearporphyrin polymer, and a porphyrin array. Certain particularly preferredstorage molecules include a porphyrinic macrocycle substituted at aβ-position or at a meso-position. Molecules well suited for use asstorage molecules include the molecules described herein (e.g. themolecules of Formulas I–XXVIII).

Particularly preferred methods and/or devices of this invention utilizea “fixed” electrode. Thus, in one embodiment, methods and/or devices inwhich the electrode(s) are moveable (e.g. one or ore electrodes is a“recording head”, the tip of a scanning tunneling microscope (STM), thetip of an atomic force microscope (AFM), or other forms in which theelectrode is movable with respect to the storage medium are excluded. Incertain embodiments, methods and/or devices and/or storage media, and/orstorage molecules in which the storage molecule is analkanethiolferrocene are excluded. Similarly in certain embodiments,methods and/or devices and/or storage media, in which the storagemolecules are responsive to light and/or in which the oxidation state ofa storage molecule is set by exposure to light are excluded.

In another embodiment, this invention provides an information storagemedium. The information storage medium can be used to assemble storagecells and/or the various memory devices described herein. In a preferredembodiment the storage medium comprises one or more different storagemolecules. When different species of storage molecule are present, eachspecies of storage molecule oxidation state(s) different from anddistinguishable from the oxidation state(s) of the other species ofstorage molecule comprising the storage medium. In preferredembodiments, the storage molecule(s) include a porphyrinic macrocycle, ametallocene, a linear polyene, a cyclic polyene, aheteroatom-substituted linear polyene, a heteroatom-substituted cyclicpolyene, a tetrathiafulvalene, a tetraselenafulvalene, a metalcoordination complex, a buckyball, a triarylamine, a1,4-phenylenediamine, a xanthene, a flavin, a phenazine, aphenothiazine, an acridine, a quinoline, a 2,2′-bipyridyl, a4,4′-bipyridyl, a tetrathiotetracene, or a peri-bridged naphthalenedichalcogenide. In even more preferred embodiment, the storagemolecule(s) include a porphyrin, an expanded porphyrin, a contractedporphyrin, a ferrocene, a linear porphyrin polymer, or a porphyrinarray. Preferred storage molecules contain two or more covalently linkedredox-active subunits. In various preferred embodiments, the storagemolecules include any of the storage molecules as described herein (e.g.the molecules of Formulas I–XXVIII).

In still another embodiment this invention provides a collection ofmolecules for the production of a data storage medium. A preferredcollection comprises a plurality of storage molecules wherein eachspecies of storage molecule has an oxidation state different from anddistinguishable from the oxidation states of the other species ofstorage molecules comprising the collection. In various preferredembodiments, the storage molecules include any of the storage moleculesas described herein (e.g. the molecules of Formulas I–XXVIII).

This invention also provides particularly preferred molecules for thestorage of information (storage molecules). The molecules preferablyhave at least one non-neutral oxidation state and more preferably haveat least two different and distinguishable non-neutral oxidation states.In various preferred embodiments, the storage molecules include any ofthe storage molecules as described herein (e.g. the molecules ofFormulas I–XXVIII).

This invention also provides methods of storing data. The methodsinvolve i) providing an apparatus, e.g., comprising one or more storagecells as described herein; and ii) applying a voltage to the electrodeat sufficient current to set an oxidation state of said storage medium(the storage medium comprising one or more storage cells). In preferredembodiments, the voltage ranges is less than about 5 volts, morepreferably less than about 2 volts, and most preferably less than about1 or less than about 0.5 volts. The voltage can be the output of anyconvenient voltage source (e.g. output of an integrated circuit, powersupply, logic gate, potentiostat, microprocessor (CPU), etc.) that canprovide a voltage/signal for writing, reading, or refreshing the storagecell(s).

The method can further involve detecting the oxidation state of thestorage medium and thereby reading out the data stored therein. Thedetection (read) can optionally involve refreshing the oxidation stateof the storage medium (particularly in static-hole devices). The read(detecting) can involve analyzing a readout signal in the time orfrequency domain and can thus involve performing a Fourier transform onthe readout signal. The detection can be by any of a variety of methodsincluding, but not limited to a voltammetric method. One particularlypreferred readout utilizes impedance spectroscopy. The readout(detecting) can involve exposing the storage medium to an electric fieldto produce an electric field oscillation having characteristic frequencyand detecting the characteristic frequency. In preferred embodiments,the storage cells used in the methods of this invention have storagemedia comprising one or more of the storage molecules described herein(e.g. the molecules of Formulas I–XXVIII).

This invention additionally provides the memory devices of thisinvention (e.g. memory cells) in a computer system. In addition computersystems utilizing the memory devices of this invention are provided.Preferred computer systems include a central processing unit, a display,a selector device, and a memory device the storage devices (e.g. storagecells) of this invention.

DEFINITIONS

The term “oxidation” refers to the loss of one or more electrons in anelement, compound, or chemical substituent/subunit. In an oxidationreaction, electrons are lost by atoms of the element(s) involved in thereaction. The charge on these atoms must then become more positive. Theelectrons are lost from the species undergoing oxidation and soelectrons appear as products in an oxidation reaction. An oxidation istaking place in the reaction Fe²⁺(aq)→Fe³⁺(aq)+e⁻ because electrons arelost from the species being oxidized, Fe²⁺(aq), despite the apparentproduction of electrons as “free” entities in oxidation reactions.Conversely the term reduction refers to the gain of one or moreelectrons by an element, compound, or chemical substituent/subunit.

An “oxidation state” refers to the electrically neutral state or to thestate produced by the gain or loss of electrons to an element, compound,or chemical substituent/subunit. In a preferred embodiment, the term“oxidation state” refers to states including the neutral state and anystate other than a neutral state caused by the gain or loss of electrons(reduction or oxidation).

The term “multiple oxidation states” means more than one oxidationstate. In preferred embodiments, the oxidation states may reflect thegain of electrons (reduction) or the loss of electrons (oxidation).

The terms “different and distinguishable” when referring to two or moreoxidation states means that the net charge on the entity (atom,molecule, aggregate, subunit, etc.) can exist in two different states.The states are said to be “distinguishable” when the difference betweenthe states is greater than thermal energy at room temperature (e.g. 0°C. to about 40° C.).

The term “electrode” refers to any medium capable of transporting charge(e.g. electrons) to and/or from a storage molecule. Preferred electrodesare metals or conductive organic molecules. The electrodes can bemanufactured to virtually any 2-dimensional or 3-dimensional shape (e.g.discrete lines, pads, planes, spheres, cylinders, etc.).

The term “fixed electrode” is intended to reflect the fact that theelectrode is essentially stable and unmovable with respect to thestorage medium. That is, the electrode and storage medium are arrangedin an essentially fixed geometric relationship with each other. It is ofcourse recognized that the relationship alters somewhat due to expansionand contraction of the medium with thermal changes or due to changes inconformation of the molecules comprising the electrode and/or thestorage medium. Nevertheless, the overall spatial arrangement remainsessentially invariant. In a preferred embodiment this term is intendedto exclude systems in which the electrode is a movable “probe” (e.g. awriting or recording “head”, an atomic force microscope (AFM) tip, ascanning tunneling microscope (STM) tip, etc.).

The term “working electrode” is used to refer to one or more electrodesthat are used to set or read the state of a storage medium and/orstorage molecule.

The term “reference electrode” is used to refer to one or moreelectrodes that provide a reference (e.g. a particular referencevoltage) for measurements recorded from the working electrode. Inpreferred embodiments, the reference electrodes in a memory device ofthis invention are at the same potential although in some embodimentsthis need not be the case.

The term “electrically coupled” when used with reference to a storagemolecule and/or storage medium and electrode refers to an associationbetween that storage medium or molecule and the electrode such thatelectrons move from the storage medium/molecule to the electrode or fromthe electrode to the storage medium/molecule and thereby alter theoxidation state of the storage medium/molecule. Electrical coupling caninclude direct covalent linkage between the storage medium/molecule andthe electrode, indirect covalent coupling (e.g. via a linker), direct orindirect ionic bonding between the storage medium/molecule and theelectrode, or other bonding (e.g. hydrophobic bonding). In addition, noactual bonding may be required and the storage medium/molecule maysimply be contacted with the electrode surface. There also need notnecessarily be any contact between the electrode and the storagemedium/molecule where the electrode is sufficiently close to the storagemedium/molecule to permit electron tunneling between the medium/moleculeand the electrode.

The term “redox-active unit” or “redox-active subunit” refers to amolecule or component of a molecule that is capable of being oxidized orreduced by the application of a suitable voltage.

The term “subunit”, as used herein, refers to a redox-active componentof a molecule.

The terms “storage molecule” or “memory molecule” refer to a moleculehaving one or more oxidation states that can be used for the storage ofinformation (e.g. a molecule comprising one or more redox-activesubunits). Preferred storage molecules have two or more different anddistinguishable non-neutral oxidation states.

The term “storage medium” refers to a composition comprising two or morestorage molecules. The storage medium can contain only one species ofstorage molecule or it can contain two or more different species ofstorage molecule.

The term “storage medium” as used herein refers to a collection ofstorage molecules. Preferred storage media comprise a multiplicity (atleast 2) of different and distinguishable (preferably non-neutral)oxidation states. The multiplicity of different and distinguishableoxidation states can be produced by the combination of different speciesof storage molecules, each species contributing to said multiplicity ofdifferent oxidation states and each species having a single non-neutraloxidation state. Alternatively or in addition, the storage medium cancomprise one or more species of storage molecule having a multiplicityof non-neutral oxidation states. The storage medium can containpredominantly one species of storage molecule or it can contain a numberof different storage molecules. The storage media can also includemolecules other than storage molecules (e.g. to provide chemicalstability, suitable mechanical properties, to prevent charge leakage,etc.).

The term “electrochemical cell” consists minimally of a referenceelectrode, a working electrode, a redox-active medium (e.g. a storagemedium), and, if necessary, some means (e.g., a dielectric) forproviding electrical conductivity between the electrodes and/or betweenthe electrodes and the medium. In some embodiments, the dielectric is acomponent of the storage medium.

The terms “memory element”, “memory cell”, or “storage cell” refer to anelectrochemical cell that can be used for the storage of information.Preferred “storage cells” are discrete regions of storage mediumaddressed by at least one and preferably by two electrodes (e.g. aworking electrode and a reference electrode). The storage cells can beindividually addressed (e.g. a unique electrode is associated with eachmemory element) or, particularly where the oxidation states of differentmemory elements are distinguishable, multiple memory elements can beaddressed by a single electrode. The memory element can optionallyinclude a dielectric (e.g. a dielectric impregnated with counterions).

The term “storage location” refers to a discrete domain or area in whicha storage medium is disposed. When addressed with one or moreelectrodes, the storage location may form a storage cell. However if twostorage locations contain the same storage media so that they haveessentially the same oxidation states, and both storage locations arecommonly addressed, they may form one functional storage cell.

Addressing a particular element refers to associating (e.g.,electrically coupling) that memory element with an electrode such thatthe electrode can be used to specifically determine the oxidationstate(s) of that memory element.

The term “storage density” refers to the number of bits per volumeand/or bits per molecule that can be stored. When the storage medium issaid to have a storage density greater than one bit per molecule, thisrefers to the fact that a storage medium preferably comprises moleculeswherein a single molecule is capable of storing at least one bit ofinformation.

The terms “read” or “interrogate” refer to the determination of theoxidation state(s) of one or more molecules (e.g. molecules comprising astorage medium).

The term “refresh” when used in reference to a storage molecule or to astorage medium refers to the application of a voltage to the storagemolecule or storage medium to re-set the oxidation state of that storagemolecule or storage medium to a predetermined state (e.g. an oxidationstate the storage molecule or storage medium was in immediately prior toa read).

The term “E_(1/2)” refers to the practical definition of the formalpotential (E°) of a redox process as defined byE=E°+(RT/nF)ln(D_(ox)/D_(red)) where R is the gas constant, T istemperature in K (Kelvin), n is the number of electrons involved in theprocess, F is the Faraday constant (96,485 Coulomb/mole), D_(ox) is thediffusion coefficient of the oxidized species and D_(red) is thediffusion coefficient of the reduced species.

A voltage source is any source (e.g. molecule, device, circuit, etc.)capable of applying a voltage to a target (e.g. an electrode).

The term “present on a single plane”, when used in reference to a memorydevice of this invention refers to the fact that the component(s) (e.g.storage medium, electrode(s), etc.) in question are present on the samephysical plane in the device (e.g. are present on a single lamina).Components that are on the same plane can typically be fabricated at thesame time, e.g., in a single operation. Thus, for example, all of theelectrodes on a single plane can typically be applied in a single (e.g.,sputtering) step (assuming they are all of the same material).

The phrase “output of an integrated circuit” refers to a voltage orsignal produced by a one or more integrated circuit(s) and/or one ormore components of an integrated circuit.

A “voltammetric device” is a device capable of measuring the currentproduced in an electrochemical cell as a result of the application of avoltage or change in voltage.

An “amperometric device” is a device capable of measuring the currentproduced in an electrochemical cell as a result of the application of aspecific potential field potential (“voltage”).

A potentiometric device is a device capable of measuring potentialacross an interface that results from a difference in the equilibriumconcentrations of redox molecules in an electrochemical cell.

A “coulometric device” is a device capable of the net charge producedduring the application of a potential field (“voltage”) to anelectrochemical cell.

An impedance spectrometer is a device capable of determining the overallimpedance of an electrochemical cell.

A “sinusoidal voltammeter” is a voltammetric device capable ofdetermining the frequency domain properties of an electrochemical cell.

The term “porphyrinic macrocycle” refers to a porphyrin or porphyrinderivative. Such derivatives include porphyrins with extra ringsortho-fused, or ortho-perifused, to the porphyrin nucleus, porphyrinshaving a replacement of one or more carbon atoms of the porphyrin ringby an atom of another element (skeletal replacement), derivatives havinga replacement of a nitrogen atom of the porphyrin ring by an atom ofanother element (skeletal replacement of nitrogen), derivatives havingsubstituents other than hydrogen located at the peripheral (meso-, β-)or core atoms of the porphyrin, derivatives with saturation of one ormore bonds of the porphyrin (hydroporphyrins, e.g., chlorins,bacteriochlorins, isobacteriochlorins, decahydroporphyrins, corphins,pyrrocorphins, etc.), derivatives obtained by coordination of one ormore metals to one or more porphyrin atoms (metalloporphyrins),derivatives having one or more atoms, including pyrrolic andpyrromethenyl units, inserted in the porphyrin ring (expandedporphyrins), derivatives having one or more groups removed from theporphyrin ring (contracted porphyrins, e.g., corrin, corrole) andcombinations of the foregoing derivatives (e.g. phthalocyanines,sub-phthalocyanines, and porphyrin isomers). Preferred porphyrinicmacrocycles comprise at least one 5-membered ring.

The term porphyrin refers to a cyclic structure typically composed offour pyrrole rings together with four nitrogen atoms and two replaceablehydrogens for which various metal atoms can readily be substituted. Atypical porphyrin is hemin.

The term “multiporphyrin array” refers to a discrete number of two ormore covalently-linked porphyrinic macrocycles. The multiporphyrinarrays can be linear, cyclic, or branched.

A linker is a molecule used to couple two different molecules, twosubunits of a molecule, or a molecule to a substrate.

A substrate is a, preferably solid, material suitable for the attachmentof one or more molecules. Substrates can be formed of materialsincluding, but not limited to glass, plastic, silicon, minerals (e.g.quartz), semiconducting materials, ceramics, metals, etc.

The term “odd hole oxidation state”, refers to the case where the numberof electron equivalents added or removed from a molecule or molecules isnot an integer multiple of the number of redox-active (e.g. oxidizableor reducable) subunits in the molecule or molecules.

The phrase “hole hopping” refers to the exchange of oxidation statesbetween subunits of thermodynamically similar potentials.

The term “aryl” refers to a compound whose molecules have the ringstructure characteristic of benzene, naphthalene, phenanthrene,anthracene, etc. (i.e., either the 6-carbon ring of benzene or thecondensed 6-carbon rings of the other aromatic derivatives). Forexample, and aryl group may be phenyl (C₆H₃) or naphthyl (C₁₀H₉). It isrecognized that the aryl, while acting as substituent can itself haveadditional substituents (e.g. the substituents provided for S^(n) in thevarious Formulas herein).

The term “alkyl” refers to a paraffinic hydrocarbon group which may bederived from an alkane by dropping one hydrogen from the formula.Examples are methyl (CH₃—), ethyl (C₂H₅—), propyl (CH₃CH₂CH₂—),isopropyl ((CH₃)₂CH₃—).

The term “halogen” refers to one or the electronegative elements ofgroup VIIA of the periodic table (fluorine, chlorine, bromine, iodine,astatine).

The term “nitro” refers to the NO₂ group.

The term “amino” refers to the NH₂ group.

The term “perfluoroalkyl” refers to an alkyl group where every hydrogenatom is replaced with a fluorine atom.

The term “perfluoroaryl” refers to an aryl group where every hydrogenatom is replaced with a fluorine atom.

The term “pyridyl” refers to an aryl group where one CH unit is replacedwith a nitrogen atom.

The term “cyano” refers to the —CN group.

The term “thiocyanato” refers to the —SCN group.

The term “sulfoxyl” refers to a group of composition RS(O)— where R issome alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group.Examples include, but are not limited to methylsulfoxyl, phenylsulfoxyl,etc.

The term “sulfonyl” refers to a group of composition RSO₂— where R issome alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group.Examples include, but are not limited to methylsulfonyl, phenylsulfonyl,p-toluenesulfonyl, etc.

The term “carbamoyl” refers to the group of composition R¹(R²)NC(O)—where R¹ and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl,or perfluoroaryl group. Examples include, but are not limited toN-ethylcarbamoyl, N,N-dimethylcarbamoyl, etc.

The term “amido” refers to the group of composition R¹CON(R²)— where R¹and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl, orperfluoroaryl group. Examples include, but are not limited to acetamido,N-ethylbenzamido, etc.

The term “acyl” refers to an organic acid group in which the OH of thecarboxyl group is replaced by some other substituent (RCO—). Examplesinclude, but are not limited to acetyl, benzoyl, etc

In preferred embodiments, when a metal is designated by “M” or “M^(n)”,where n is an integer, it is recognized that the metal may be associatedwith a counterion.

The term “substituent” as used in the formulas herein, particularlydesignated by S or S^(n) where n is an integer, in a preferredembodiment refer to redox-active groups (subunits) that can be used toadjust the redox potential(s) of the subject compound. Preferredsubstituents include, but are not limited to, aryl, phenyl, cycloalkyl,alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl,pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl,sulfonyl, amido, and carbamoyl. In preferred embodiments, a substitutedaryl group is attached to a porphyrin or a porphyrinic macrocycle, andthe substituents on the aryl group are selected from the groupconsisting of aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy,alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato,nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, amido, andcarbamoyl.

Particularly preferred substituents include, but are not limited to,4-chlorophenyl, 3-acetamidophenyl, 2,4-dichloro-4-trifluoromethyl).Preferred substituents provide a redox potential range of less thanabout 5 volts, preferably less than about 2 volts, more preferably lessthan about 1 volt.

The phrase “provide a redox potential range of less than about X volts”refers to the fact that when a substituent providing such a redoxpotential range is incorporated into a compound, the compound into whichit is incorporated has an oxidation potential less than or equal to Xvolts, where X is a numeric value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a basic molecular memory unit “storage cell” of thisinvention. The basic memory device, a “storage cell” 100 comprises aworking electrode 101 electrically coupled to a storage medium 102comprising a multiplicity of storage molecules 105. The storage celloptionally includes an electrolyte 107 and a reference electrode 103.The storage medium has a multiplicity of different and distinguishableoxidation states, preferably a multiplicity of different anddistinguishable non-neutral oxidation states, and can change oxidation(charge) state when a voltage or signal is applied thereby adding orremoving one or more electrons.

FIG. 2 illustrates the disposition of the storage cell(s) of thisinvention on a chip.

FIG. 3 illustrates a preferred chip-based embodiment of this invention.A two-level chip is illustrated showing working electrodes 101,orthogonal reference electrodes 103, and storage elements 104.

FIG. 4. The three-dimensional architecture of a single memory storagecell (memory element) on the chip.

FIG. 5 illustrates the encoding of a prototypical DHMU storage moleculeusing hole-hopping states (the double-headed arrows indicate holehopping).

FIG. 6 illustrates porphyrin mono-thiols for attachment to a metal(e.g., gold) electrode.

FIG. 7 illustrates the modular synthesis of a SHMU storage molecule.

FIG. 8 illustrates a representative synthesis of a DHMU storagemolecule. Three porphyrin building blocks are prepared and metalatedwith magnesium or zinc. The synthetic strategy builds the two arms ofthe DHMU storage molecule separately, which are then coupled in thepenultimate step of the synthesis. Each arm is constructed via twoPd-mediated couplings, yielding the respective trimers. One trimer isiodinated at the ethyne, then joined with the other trimer in aheterocoupling process to form the H-like structure.

FIG. 9 illustrates writing to a molecular memory of this invention. Inpreferred embodiments, this is accomplished through the application ofvery short (e.g., microsecond) pulses applied at a voltage sufficient tooxidize a storage medium (e.g., a porphyrin) to the appropriate redoxstate as summarized in this figure. Thus, each redox state of thecomposite multiunit nanostructure (e.g. porphyrinic array) can beindependently accessed to provide one bit of resolution. This can beaccomplished via the electrochemical oxidation of the molecule instepwise increments.

FIG. 10 illustrates a frequency domain spectrum of the faradaic SVresponse. Note that the numerous harmonic frequency components depend onmany of the same voltammetric parameters (e.g., E°, E_(switch), scanrate, number of electrons, etc.) that govern the response observed incyclic voltammetry, and can be easily isolated in the frequency domain.

FIG. 11 illustrates a sinusoidal voltammetry system suitable for readoutof the memory devices of this invention.

FIG. 12 illustrates a computer system embodying the memory devicesdescribed herein. Typically the memory device will be fabricated as asealed “chip”. Ancillary circuitry on the chip and/or in the computerpermits writing bits into the memory and retrieving the writteninformation as desired.

FIG. 13 illustrates the memory devices of this invention integrated intoa standard computer architecture or computer system 200.

FIG. 14 illustrates synthesis scheme 1 for the synthesis of latentbenzaldehydes with various protecting groups for the p-thiol moiety.These are used in the synthesis of thiol-substituted porphyrins.

FIG. 15 illustrates synthesis scheme 2 for the synthesis ofbenzaldehydes with protected thiol groups. These are used in thesynthesis of thiol-derivatized porphyrins.

FIG. 16 illustrates synthesis scheme 3 for the synthesis of metallo-freeand zinc porphyrins each bearing three mesityl groups and one protectedp-thiophenyl group.

FIG. 17 illustrates synthesis scheme 4 for the synthesis of a zincporphyrin bearing three mesityl groups and one free thiol group.

FIG. 18 illustrates synthesis scheme 5 for the synthesis of a magnesiumporphyrin bearing three mesityl groups and one p-mercaptophenyl group.

FIG. 19 illustrates synthesis scheme 6 for the synthesis of metallo-freeand zinc porphyrins each bearing three groups to tune the oxidationpotential and one free or protected p-thiophenyl group.

FIG. 20 illustrates synthesis scheme 7 for the synthesis of metallo-freeand zinc porphyrins bearing four m-(thiocyanatomethyl)phenyl groups forhorizontal orientation on a gold surface.

FIG. 21 illustrates synthesis scheme 8 for the synthesis of metallo-freeand zinc porphyrins bearing two m-(thiocyanatomethyl)phenyl groups forhorizontal orientation on a gold surface.

FIG. 22 illustrates synthesis scheme 9 for the synthesis of metallo-freeand zinc porphyrins bearing four m-(S-acetylthiomethyl)phenyl groups forhorizontal orientation on a gold surface.

FIG. 23 illustrates the writing of bits on a porphyrin monolayer havingtwo non-neutral oxidation states. A plot of current versus time at 3applied voltages is illustrated. At 0–300 mV, no bit is set and the plotprovides a background signal. At 500–800 mV and at 800–1100 mV the firstand second bits are written, respectively.

FIG. 24 illustrates the read/write of a monomeric porphyrin. Current isplotted as a function of potential.

FIG. 25 illustrates background-subtracted faradaic read current.

DETAILED DESCRIPTION

This invention provides novel high density memory devices that areelectrically addressable permitting effective reading and writing, thatprovide a high memory density (e.g., 10¹⁵ bits/cm³), that provide a highdegree of fault tolerance, and that are amenable to efficient chemicalsynthesis and chip fabrication. The devices are intrinsically latchable,defect tolerant, and support destructive or non-destructive read cycles.

One embodiment of this invention is illustrated in FIG. 1. The basicmemory device, a “storage cell” 100 comprises a working electrode 101electrically coupled to a storage medium 102 comprising a multiplicityof storage molecules 105. The storage cell optionally includes anelectrolyte 107 and a reference electrode 103. The storage medium has amultiplicity of different and distinguishable oxidation states,preferably a multiplicity of different and distinguishable non-neutraloxidation states, and can change oxidation (charge) state when a voltageor signal is applied thereby adding or removing one or more electrons.Each oxidation state represents a particular bit. Where the storagemedium supports eight different and distinguishable oxidation states itstores one byte.

The storage medium remains in the set oxidation state until anothervoltage is applied to alter that oxidation state. The oxidation state ofthe storage medium can be readily determined using a wide variety ofelectronic (e.g. amperometric, coulometric, voltammetric) methodsthereby providing rapid readout.

The storage medium comprises molecules having a single oxidation stateand/or molecules having multiple different and distinguishablenon-neutral oxidation states. Thus, for example, in one embodiment, thestorage medium can comprise eight different species of storage moleculeseach having one non-neutral oxidation state and thereby store one byte.In another embodiment, the storage medium can comprise one species ofmolecule that has eight different and distinguishable oxidation statesand store one byte in that manner as well. As explained herein, a largenumber of different molecules having different numbers of oxidationstates can be used for the storage medium.

Because molecular dimensions are so small (on the order of angstroms)and individual molecules in the devices of this invention can storemultiple bits, the storage devices of this invention therefore offerremarkably high storage densities (e.g. >10¹⁵ bits/cm³).

Moreover, unlike prior art, the devices of this invention are capable ofa degree of self-assembly and hence easily fabricated. Because thedevices are electrically (rather than optically) addressed, and becausethe devices utilize relatively simple and highly stable storageelements, they are readily fabricated utilizing existing technologiesand easily incorporated into electronic devices. Thus, the molecularmemory devices of this invention have a number of highly desirablefeatures:

Because the storage medium of the devices described herein iselectrically-addressed, the devices are amenable to the construction ofa multilayered chip architecture. An architecture compatible with such athree-dimensional structure is essential to achieve the objective of10¹⁵ bits/cm³. In addition, because writing and reading is accomplishedelectrically, many of the fundamental problems inherent with photonicsare avoided. Moreover, electrical reading and writing is compatible withexisting computer technology for memory storage.

In addition, the devices of this invention achieve a high level ofdefect tolerance. Defect tolerance is accomplished through the use ofclusters of molecules (up to several million in a memory cell). Thus,the failure of one or a few molecules will not alter the ability to reador write to a given memory cell that constitutes a particular bit ofmemory. In preferred embodiments, the basis for memory storage relies onthe oxidation state(s) of porphyrins or other porphyrinic macrocycles ofdefined energy levels. Porphyrins and porphyrinic macrocycles are wellknown to form stable radical cations. Indeed, the oxidation andreduction of porphyrins provide the foundation for the biologicalprocesses of photosynthesis and respiration. Porphyrin radical cationscan be formed chemically on the benchtop exposed to air. We know of noother class of molecules with such robust electroactive properties.

Preferred storage molecules of this invention molecule (e.g., SHMU orDHMU) can hold multiple holes, corresponding to multiple bits. Incontrast, the dyes (photochromic, electrochromic, redox) and molecularmachines are invariably bistable elements. Bistable elements existeither in a high/low state and hence can only store a single bit. TheSHMU and DHMU are unique molecular nanostructures providing resilientstorage of multiple bits.

Reading can be accomplished non-destructively or destructively asrequired in different chip applications. The speed of reading isconservatively estimated to lie in the MHz to GHz regime. Memory storageis inherently latchable due to the stability of the porphyrin or otherporphyrinic macrocycle radical cations. Oxidation of the porphyrins orother porphyrinic macrocycles can be achieved at relatively lowpotential (and at predesignated potentials through synthetic design),enabling memory storage to be achieved at very low power. Porphyrins andporphyrin radical cations are stable across a broad range oftemperatures, enabling chip applications at low temperature, roomtemperature, or at elevated temperatures.

Fabrication of the devices of this invention relies on known technology.The synthesis of the storage media takes advantage of establishedbuilding block approaches in porphyrin and other porphyrinic macrocyclechemistry. Synthetic routes have been developed to make the porphyrinand porphyrinic macrocycle building blocks, to join them in covalentnanostructures, and to purify them to a high level (>99%).

In preferred embodiments, the storage medium nanostructures are designedfor directed self-assembly on gold surfaces. Such self-assemblyprocesses are robust, result in the culling out of defective molecules,and yield long-range order in the surface-assembled cluster.

Porphyrin-thiols have been assembled on electroactive surfaces. Thearrays that define the addressable bits of memory can be achievedthrough conventional microfabrication techniques. The storage moleculesare self-assembled onto these electrode arrays and attached to the goldsurface using conventional dipping methods.

I. Uses of the Storage Device.

One of ordinary skill in the art will appreciate that the memory devicesof this invention have wide applicability in specialized andgeneral-purpose computer systems. Of course commercial realization ofthe device(s) will be facilitated by the adoption of computerarchitecture standards compatible with this technology. In addition,commercial adoption of this technology will be facilitated by the use ofother molecular electronic components that will serve as on-chip buffersand decoders (that is, molecular logic gates), and the like. Inaddition, commercialization will be facilitated by the development of afull manufacturing infrastructure.

Regardless, prior to the development of a fully integrated design andmanufacturing platform for molecular electronic information storage andtransfer, even early generation prototype molecular memory devicesdescribed herein have utility in highly specialized military and/orstealthy applications. For example, a prototype 1024/512-bit molecularmemory device has sufficient capacity to hold a substantial base ofpersonal and/or other proprietary information. This information could betransported anywhere in the world virtually undetected owing to theextremely small size of the device. If detected, the memory device iseasily erased simply by applying a low potential reverse bias currentacross all memory cells. This protection mechanism can be readilyincorporated into any type of transport architecture designed for thememory device.

The memory devices of this invention have sufficient capacity to holdtargeting information that could be used in miniaturized, expendabledelivery vehicles. Even a memory device that degrades upon multiple readcycles is extremely useful if the number of read cycles is highlylimited (perhaps only one). A memory device that degrades upon multipleread cycles or simply with time is also useful in applications wherelong-term data persistence is not needed or is strategically unwise.Thus, numerous strategically important applications for early generationmemory devices present themselves. Successes of the memory devices inthese applications will foster even more rapid full-scalecommercialization of the technology.

II. Architecture of the Storage Device.

The basic storage cell (electrode(s) and storage medium) of thisinvention can be incorporated into a functional device in a wide varietyof configurations. One chip-based embodiment of this invention isillustrated in FIG. 2. As illustrated in FIG. 2 the storage medium 102is disposed in a number of storage locations 104. Each storage locationis addressed by a working electrode 101 and a reference electrode 103 sothat the storage medium 102 combined with the electrodes forms a storagecell 100 at each storage location.

One particularly preferred chip-based embodiment is illustrated in FIG.3. In the illustrated embodiment, a plurality of working electrodes 101and reference electrodes 103 are illustrated each addressing storagemedia 102 localized at discrete storage locations thereby forming aplurality of storage cells 100. Multiple storage cells can be associatedwith a single addressing electrode as long as oxidation states of thestorage cells are distinguishable from each other. It should be notedthat this forms a functional definition of a storage cell. Where twodiscrete areas of storage medium are addressed by the same electrode(s)if the storage media comprise the same species of storage molecule thetwo discrete areas will functionally perform as a single storage cell,i.e. the oxidation states of both locations will be commonly set, and/orread, and/or reset. The added storage location, however, will increasethe fault tolerance of the storage cell as the functional storage cellwill contain more storage molecules. In another embodiment, eachindividual storage cell is associated with a single addressingelectrode.

In preferred embodiments, the storage medium comprising the storagecells of a memory device are all electrically coupled to one or morereference electrodes. The reference electrode(s) can be provided asdiscrete electrodes or as a common backplane.

The chip illustrated in FIG. 3 has two levels of working electrodes andhence two levels of storage cells 100 (with numerous storage cells oneach level). Of course, the chip can be fabricated with a single levelof electrodes and memory element or literally hundreds or thousands ofdifferent levels of storage cell(s), the thickness of the chip beinglimited essentially by practical packaging and reliability constraints.

In particularly preferred embodiments, a layer of dielectric materialoptionally imbedded with counterions to ensure electrical connectivitybetween the working and reference electrode(s) and stability of thecationic species in the absence of applied potential (latching) isdisposed in the storage cell. In some embodiments, the dielectricmaterial can be incorporated into the storage medium itself.

While, in some preferred embodiments, feature sizes are rather large(e.g. memory elements approximately (10×10×10 μm) and electrodethickness ˜200 nm, feature size can be reduced at will so that featuresizes are comparable to those in conventional silicon-based devices(e.g., 50 nm–100 nm on each axis).

In a preferred embodiment, the storage device includes: (1) A goldworking electrode (e.g., 200 nm thick), deposited on a nonconductingbase, and line-etched to achieve electrode widths of 10's to 100's ofnm. (2) A monolayer of self-assembled porphyrinic nanostructures(storage molecules 105) attached to the gold surface via the sulfur atomof the thiophenol group. (3) A 100-nm thick layer of dielectric material107 embedded with counterions to ensure electrical connectivity to thereference electrode and stability of the cationic species in the absenceof applied potential (latching). (4) A 200-nm thick nonpolarizablereference electrode 103 line etched in the same fashion as those of theworking electrode 101, but assembled with lines orthogonal to the latterelectrode. (5) A mirror image construct that utilizes the same referenceelectrode. Thus, in one embodiment, the three-dimensional architectureof a single memory storage location (memory element) on the chip willlook as indicated in FIG. 4.

While the discussion herein of electrodes is with respect to goldelectrodes, it will be recognized that numerous other materials will besuitable. Thus, electrode materials include, but are not limited togold, silver, copper, other metals, metal alloys, organic conductors(e.g. doped polyacetylene, doped polythiophene, etc.), nanostructures,crystals, etc.

Similarly, the substrates used in the fabrication of devices of thisinvention include, but are not limited to glasses, silicon, minerals(e.g. quartz), plastics, ceramics, membranes, gels, aerogels, and thelike.

III. Fabrication and Characterization of the Storage Device.

A) Fabrication.

The memory devices of this invention can be fabricated using standardmethods well known to those of skill in the art. In a preferredembodiment, the electrode layer(s) are applied to a suitable substrate(e.g. silica, glass, plastic, ceramic, etc.) according to standard wellknown methods (see, e.g., Choudhury (1997) The Handbook ofMicrolithography, Micromachining, and Microfabrication, Soc.Photo-Optical Instru. Engineer, Bard & Faulkner (1997) Fundamentals ofMicrofabrication). In addition, examples of the use of micromachiningtechniques on silicon or borosilicate glass chips can be found in U.S.Pat. Nos. 5,194,133, 5,132,012, 4,908,112, and 4,891,120.

In one preferred embodiment a metal layer is beam sputtered onto thesubstrate (e.g., a 10 nm thick chromium adhesion layer is sputtered downfollowed by a 200 nm thick layer of gold). Then maskless laser ablationlithography (see below), performed e.g., with a Nd:YAG laser, is used tocreate features with micron dimensions, or with an excimer laser tocreate features of nanometer dimensions) will create an array ofparallel lines of conductor (e.g., gold), used as the working electrodeswith dimensions ranging between a few microns to a tens of nanometers;

Once the electrode array is formed, the entire array, or portions of thearray, or individual electrodes are wetted (e.g. immersed or spotted)with one or more solutions of the appropriate derivatized storage media(e.g. thiol-substituted porphyrin nanostructures), and the constituentsof the memory medium (e.g., monomeric porphyrin subunits) self-assembleon the micro-sized gold arrays to form the memory elements. It will beappreciated that different solutions can be applied to different regionsof the electrode array to produce storage cells comprising differentstorage medium. Methods of spotting different reagents on surfaces (e.g.on glass surfaces) at densities up to tens of thousands of differentspecies/spots per cm² are known (see, e.g., U.S. Pat. No. 5,807,522).

Then a suitable electrolyte layer (e.g. a thin layer of Nafion polymer)approximately 1 nm to 1000 nm, preferably about 100 nm to about 500 nm,more preferably about 10 nm to about 100 nm and most preferably aboutone hundred nanometers thick) will be cast over the entire surface ofthe chip. This polymer serves to hold the electrolyte forelectrochemical reaction. Finally, the entire chip is coated with alayer (e.g., 10 nm to about 1000 nm, more preferably 100 nm to about 300nm and most preferably about 200 nm of conducting material (e.g. silver)which acts as a reference electrode 103.

The chip is then turned 90 degrees, and maskless laser ablationlithography will be performed again to create a second array of parallellines that are perpendicular to the original set. This forms a threedimensional array of individual memory elements, where each element isformed by the intersection of these two perpendicular linear arrays (seeFIG. 4).

Each individual element can be addressed by selecting the appropriate Xand Y logic elements, corresponding to one gold working electrode andone reference electrode separated by the Nafion polymer/electrolytelayer. Since this structure is inherently three dimensional, it shouldbe possible to extend the array into the Z-direction, creating a 3-Darray of memory elements as large as it is feasible to connect to.

These structures are initially created on the micron scale. It ispossible to decrease the size of these structures to sub-microndimensions. It is possible to create these structures on a scale similarto silicon microstructures created with conventional nanolithographictechniques (i.e. 100–200 nm). This would allow the interfacing of thememory elements with conventional silicon-based semiconductorelectronics.

In the laser-ablation lithography discussed above, coherent light issent through a beam splitter (50% transmittance) and reflected by amirror to make two nearly parallel identical beams (Rosenwald et al.(1998) Anal. Chem., 70: 1133–1140). These beams are sent through e.g., a50 cm focal length lens for ease in focusing to a common point. Theplacement of the beams is fine-tuned to allow complete overlap of themode structure of the laser spot. Higher order interference patterns areminimized through the use of high quality optics (1/10 wave surfaceflatness). This ensures that the variation between intensity maxima andminima in the first order will be several orders of magnitude largerthan those formed with second and higher orders. This produces awell-defined pattern of lines across the electrode surface, where thespacing between points of positive interference (D) can be approximatedby the Bragg Equation: nλ=2D sin(θ/2), where λ=wavelength, θ=anglebetween the beams, and n is order. For example, when a Nd:YAG is used at1064 nm, the recombination of the two beams in this manner generates aninterference pattern with ˜2 micron spacing when the angle between the 2beams is 15°. The interference pattern spacing can easily be changed bymodifying the angle between the beams. Attenuation of the beam wasaccomplished by inserting one or more neutral density filters before thebeam splitter. In this way, the exposure of the gold layer to the Nd-YAGinterference pattern can be performed at different beam attenuations toproduce power densities between 1 and 100 MW/cm².

B) Electrically Coupling Storage Medium to Electrode.

In the storage devices of this invention, the storage medium iselectrically coupled to one or more electrodes. The term “electricalcoupling” is used to refer to coupling schemes that permit the storagemedium to gain or lose electrons to the electrode. The coupling can be adirect attachment of the storage medium to the electrode, or an indirectattachment (e.g. via a linker). The attachment can be a covalentlinkage, an ionic linkage, a linkage driven by hydrogen bonding or caninvolve no actual chemical attachment, but simply a juxtaposition of theelectrode to the storage medium. In some embodiments, the electrode canbe some distance (e.g, about 5 Å to about 50 Å) from the storage mediumand electrical coupling can be via electron tunneling.

In some preferred embodiments, a “linker” is used to attach themolecule(s) of the storage medium to the electrode. The linker can beelectrically conductive or it can be short enough that electrons canpass directly or indirectly between the electrode and a molecule of thestorage medium.

The manner of linking a wide variety of compounds to various surfaces iswell known and is amply illustrated in the literature. Means of couplingthe molecules comprising the storage medium will be recognized by thoseof skill in the art. The linkage of the storage medium to a surface canbe covalent, or by ionic or other non-covalent interactions. The surfaceand/or the molecule(s) may be specifically derivatized to provideconvenient linking groups (e.g. sulfur, hydroxyl, amino, etc.).

The linker can be provided as a component of the storage mediummolecule(s) or separately. Linkers, when not joined to the molecules tobe linked are often either hetero- or homo-bifunctional molecules thatcontain two or more reactive sites that may each form a covalent bondwith the respective binding partner (i.e. surface or storage mediummolecule). When provided as a component of a storage molecule, orattached to a substrate surface, the linkers are preferably spacershaving one or more reactive sites suitable for bonding to the respectivesurface or molecule.

Linkers suitable for joining molecules are well known to those of skillin the art and include, but are not limited to any of a variety of, astraight or branched chain carbon linker, or a heterocyclic carbonlinker, amino acid or peptide linkers, and the like. Particularlypreferred linkers include, but are not limited to 4,4′-diphenylethyne,4,4′-diphenylbutadiyne, 4,4′-biphenyl, 1,4-phenylene, 4,4′-stilbene,1,4-bicyclooctane, 4,4′-azobenzene, 4,4′-benzylideneaniline, and4,4″-terphenyl. Linkers include molecules that join one or moremolecules of the storage medium to the electrode(s).

C) Addressing the Memory Cells.

Addressing of the storage cell(s) in the devices of this invention isrelatively straightforward. In a simple approach a discrete pair ofelectrodes (one working and one reference electrode) can be connected toevery storage cell. Individual reference electrodes, however are notrequired and can be replaced with one or more common referenceelectrodes connected to all or to a subset of all of the storageelements in a particular device. Alternatively, the common referenceelectrodes can be replaced with one or more conductive “backplanes” eachcommunicating to all, or to a subset, of the storage cells in aparticular device.

Where the storage cells contain identical storage media, each storagecell is preferably addressed with a separate working electrode so thatthe storage (oxidation) states of the storage cells can be distinguishedfrom each other. Where the storage cells contain different storage mediasuch that the oxidation states of one storage cell is different anddistinguishable from the oxidation states of another storage cell, thestorage cells are preferably addressed by a common working electrodethereby reducing the number of electrodes in a device.

In one preferred embodiment, the storage devices of this inventioncontain (64, 128, 256, 512, 1024 or more storage locations per layer(64, 128, 256, 512, 1024 or more locations in the mirror imagearchitecture) with each location capable of holding a multiple-bit SHMUor DHMU word. Accordingly, a preferred 1024-bit SHMU or a preferred512-bit DHMU chip will contain 8 wiring interconnects on each of thethree electrode grids in the 3-dimensional WPDRDPW architectureillustrated in FIG. 4.

D) Characterization of the Memory Device.

The performance (e.g. operating characteristics) of the memory devicesof this invention is characterized by any of a wide variety of methods,most preferably by electrochemical methods (amperometry, sinusoidalvoltammetry and impedance spectroscopy, see, e.g., Howell et al. (1986)Electroanal. Chem., 209: 77–90; Singhal et al. (1997) Anal. Chem., 69:1662–1668; Schick et al. (1989) Am. Chem. Soc. 111: 1344–1350), atomicforce microscopy, electron microscopy and imaging spectroscopic methods.Surface-enhanced resonance and Raman spectroscopy are also used toexamine the storage medium on the electrodes.

Among other parameters, characterization of the memory devices (e.g.,memory cells) involves determining the number of storage mediummolecules (e.g., porphyrin arrays) required for defect-tolerantoperation. Defect tolerance includes factors such as reliably depositingthe required number of holes to write the desired digit and accuratelydetecting the numbers/hopping rates of the holes.

The long-term resistance of electron holes to charge-recombination inthe solid-phase medium of the device package is also determined. Usingthese parameters, the device architecture can be optimized forcommercial fabrication.

IV. Architecture of the Storage Medium.

The storage medium used in the devices of this invention comprises oneor more species of storage molecule. A preferred storage medium ischaracterized by having a multiplicity of oxidation states. Thoseoxidation states are provided by one or more redox-active units. Aredox-active unit refers to a molecule or to a subunit of a moleculethat has one or more discrete oxidation states that can be set byapplication of an appropriate voltage. Thus, for example, in oneembodiment, the storage medium can comprise one species of redox-activemolecule where that molecule has two or more (e.g. 8) different anddistinguishable oxidation states. Typically, but not necessarily, suchmulti-state molecules will be composed of several redox-active units(e.g. porphyrins or ferrocenes). In another exemplary embodiment, thestorage medium can comprise two or more different species of storagemolecule. Each storage molecule comprises at least one redox-activeunit, but can easily contain two or more redox-active units. Where eachspecies of storage molecule has a single, non-neutral, oxidation state,the storage medium achieves multiple bit storage by having a pluralityof such molecules where each molecule has a different anddistinguishable oxidation state (e.g. each species of molecule oxidizesat a different and distinguishable potential). Of course, each speciesof molecule can have a multiplicity of different and distinguishableoxidation states. Thus, a storage medium comprising eight differentspecies of storage molecule where each of the eight species has eightdifferent and distinguishable oxidation states, will be able to store 64(8×8) bits of information.

As indicated above, the storage medium can be broken down intoindividual, e.g., spatially segregated, storage locations. Each storageelement can have a storage medium that is the same or different from theother storage elements in the chip and/or system. Where the storageelements are of identical composition, in preferred embodiments, theyare separately addressed so that information in one element can bedistinguished from information in another element. Where the storageelements are of different composition they can be commonly addressed(where the oxidation states of the commonly addressed storage elementsare distinguishable) or they can be individually addressed.

In certain preferred embodiments the storage medium is juxtaposed to adielectric medium to insure electrical connectivity to a referencevoltage (e.g. a reference electrode, a reference backplane, etc.). Inparticularly preferred embodiments, a layer of dielectric material isimbedded with counterions to ensure electrical connectivity to thereference electrode and stability of the cationic species in the absenceof applied potential (latching) is disposed between the referenceworking electrode(s).

Dielectric materials suitable for the devices of this invention are wellknown to those of skill in the art. Such materials include, but are notlimited to nafion, cellulose acetate, polystyrene sulfonate,poly(vinylpyridine), electronically conducting polymers such aspolypyrrolic acid and polyaniline, etc.

The porphyrinic macrocycles identified herein are ideally suited formolecular based memory storage. The porphyrinic macrocycles, andespecially the porphyrins, have unique electroactive properties, awell-developed modular synthetic chemistry, and in conjunction withthiols, and other linkers described herein, undergo directedself-assembly on electroactive surfaces.

In addition, as described below, the porphyrinic macrocycles are wellsuited for the design of multi-bit storage systems. In preferredembodiments, this invention contemplates three fundamental architecturesfor the storage medium; static hole single-unit (SHSU) storage (e.g.SHSU molecules), static hole multi-unit (SHMU) storage (e.g. SHSUmolecules), and dynamic hole multi-unit (DHMU) storage (e.g. DHMUmolecules).

A) Static Hole Single Unit (SHSU) Storage.

In the simplest embodiments of this invention, the storage mediumcomprises one or more molecules wherein each molecule has onenon-neutral oxidation state. Thus, each molecule is capable of storingone bit (e.g. bit=1 when oxidized and bit=0 when neutral). A number ofdifferent species of static hole single unit storage molecules can beassembled into a single storage medium. Thus, for example a number ofdifferent ferrocenes, or a number of different porphyrins, orcombinations of porphyrin and ferrocene monomers can be combined into asingle storage medium.

In one preferred embodiment, a molecule comprising a static hole singleunit molecular memory has the formula shown in Formula I.

where L is a linker, M is a metal (e.g., Fe, Ru, Os, Co, Ni, Ti, Nb, Mn,Re, V, Cr, W), S¹ and S² are substituents independently selected fromthe group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen,alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano,thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido,amido, and carbamoyl. In preferred embodiments, a substituted aryl groupis attached to the porphyrin, and the substituents on the aryl group areselected from the group consisting of aryl, phenyl, cycloalkyl, alkyl,halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl,cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl,imido, amido, and carbamoyl. Particularly preferred substituentsinclude, but are not limited to, 4-chlorophenyl, 3-acetamidophenyl,2,4-dichloro-4-trifluoromethyl). Preferred substituents provide a redoxpotential range of less than about 2 volts. X is selected from the groupconsisting of a substrate, a reactive site that can covalently couple toa substrate, and a reactive site that can ionically couple to asubstrate. It will be appreciated that in some embodiments, L-X can bereplaced with another substituent (S³) like S¹ or S². In certainembodiments, L-X can be present or absent, and when present preferablyis 4-(2-(4-mercaptophenyl)ethynyl)phenyl, 4-mercaptomethylphenyl,4-hydroselenophenyl, 4-(2-(4-hydroselenophenyl)ethynyl)phenyl,4-hydrotellurophenyl, or 4-(2-(4-hydrotellurophenyl)ethynyl)phenyl.

The oxidation state of molecules of Formula I is determined by the metaland the substituents. Thus, particular preferred embodiments areillustrated by Formulas II–VII, (listed sequentially) below:

The ferrocenes listed above in Formulas II through VII provide aconvenient series of one-bit molecules having different anddistinguishable oxidation states. Thus the molecules of Formulas IIthrough VII have oxidation states (E_(1/2)) of +0.55 V, +0.48V, +0.39 V,+0.17 V, −0.05 V, and −0.18 V, respectively, and provide a convenientseries of molecules for incorporation into a storage medium of thisinvention. It will be appreciated that the oxidation states of themembers of the series can be routinely altered by changing the metal (M)or the substituents.

B) Static Hole Multi-unit (SHSU) Storage.

Static hole multi-unit (SHSU) molecular memories typically comprise amultiplicity of redox-active subunits. In a preferred embodiment, theredox-active subunits are covalently linked to form a single moleculeand are selected to have different and distinguishable oxidation states,preferably a multiplicity of different and distinguishable non-neutraloxidation states. Thus, in this configuration a single molecule can havemultiple (e.g. 2, 4, 8, 16, 32, 64, 128, 512 etc.) different non-neutraloxidation states.

In one particularly preferred embodiment the static hole multi-unitmolecular memory is a “static hole multiporphyrin molecular memory”(SHMMM) storage system. In this embodiment, the redox-active subunitsare porphyrinic macrocycles, most preferably porphyrins. The porphyrinscan be arranged in a wide variety of configurations (e.g. linearpolymers, branched polymers, arrays, etc.), however, linearconfigurations are well suited to the practice of this invention.

One particularly preferred linear configuration is illustrated byFormula VIII.

where S¹, S², S³, and S⁴ are substituents independently selected fromthe group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen,alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano,thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido,amido, and carbamoyl wherein said substituents provide a redox potentialrange of less than about 2 volts, M¹, M², M³, and M⁴ are independentlyselected metals (e.g., Zn, Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co, Rh,Ir, Mn, B, Al, Ga, Pb, and Sn), K¹, K², K³, K⁴, K⁵, K⁶, K⁷, K⁸, K⁹, K¹⁰,K¹¹, K¹², K¹³, K¹⁴, K¹⁵, and K¹⁶ are independently selected from thegroup consisting of N, O, S, Se, Te, and CH, J¹, J², and J³ areindependently selected linkers, L¹, L², L³, and L⁴ are present or absentand, when present are independently selected linkers, X¹, X², X³, and X⁴are independently selected from the group consisting of a substrate, areactive site that can covalently couple to a substrate, and a reactivesite that can ionically couple to a substrate, and E¹ and E² areterminating substituents independently aryl, phenyl, cycloalkyl, alkyl,halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl,cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl,imido, amido, or carbamoyl wherein said substituents provide a redoxpotential range of less than about 2 volts. In preferred embodiments,the molecule has at least two, preferably at least 4, more preferably atleast 8, and most preferably at least 16, at least 32, at least 64 or atleast 128 different and distinguishable oxidation states. In someembodiments, one or more of the linker/reactive site subunits (L¹-X¹,L²-X², L³-X³, or L⁴-X⁴), can be eliminated and replaced with asubstituent independently selected from the same group as S¹, S², S³, orS⁴.

In preferred embodiments, the substituents are selected so that themolecule illustrated by Formula XVIII has at least 2, more preferably atleast 4 and most preferably at least 8 different and distinguishableoxidation states.

In certain preferred embodiments, J¹, J², and J³ are independently4,4′-diphenylethyne, 4,4′-diphenylbutadiyne, 4,4′-biphenyl,1-4-phenylene, 4,4′-stilbene, 1,4-bicyclooctane, 4,4′-azobenzene,4,4′-benzylideneaniline, or 4,4″-terphenyl.

L¹-X¹, L²X², L³-X³, and L⁴-X⁴ are independently present or absent and,when present, can include 4-(2-(4-mercaptophenyl)ethynyl)phenyl,4-mercaptomethylphenyl, 4-hydroselenophenyl,4-(2-(4-hydroselenophenyl)ethynyl)phenyl, 4-hydrotellurophenyl, and4-(2-(4-hydrotellurophenyl)ethynyl)phenyl.

In a particularly preferred embodiment, K¹, K², K³, K⁴, K⁵, K⁶, K⁷, K⁸,K⁹, K¹⁰, K¹¹, K¹², K¹³, K¹⁴, K¹⁵, and K¹⁶ are the same, M¹ and M³ arethe same, M² and M⁴ are the same and different from M¹ and M³, S¹ and S²are the same; and S³ and S⁴ are the same and different from S¹ and S².

In a most preferred embodiment, the metals (M¹, M², M³, and M⁴) and thesubstituents (S¹, S², S³, and S⁴) are selected so that each porphyrinhas two non-neutral oxidation states. L¹-X¹, L²-X², L³-X³, and L⁴-X⁴provide convenient linkers for attaching the molecule to a substrate(e.g. an electrode). With each subunit having two oxidation states, thesubunits can be configured so that the entire molecule has 8 differentand distinguishable oxidation states. One such molecule is illustratedby Formula IX.

The porphyrin metalation state alters between Mg and Zn in proceedingfrom one end to the other. The different metalation state alters theredox characteristics of the porphyrins. In particular, magnesiumporphyrins are more easily oxidized than zinc porphyrins.Differentiation of the oxidation potentials of the left-most pair of Znand Mg porphyrins from those of the right-most pair is achieved throughthe use of different substituents (Ar², right pair; Ar¹, left pair)attached to the meso-(and/or to the β-) positions. The porphyrins arejoined via linkers (e.g. p,p′-diarylethyne linkers). These constrain theporphyrins at fixed distances from each other. In addition, eachporphyrin bears a linker (e.g., a thiol) for attachment to anelectroactive surface such as gold.

Information is stored in the SHMU storage molecule by removing electronsfrom the porphyrin constituents (leaving a hole and forming a π-cationradical (Strachan et al. (1997) J. Am. Chem. Soc., 119: 11191–11201; Liet al. (1997) J. Mater. Chem. 7: 1245–1262, and Seth et al. (1996) J.Am. Chem. Soc. 118: 11194–11207; Seth et al. (1994) J. Am. Chem. Soc.116: 10578–10592). The redox characteristics of the Zn and Mg porphyrinsin conjunction with the substituents Ar¹ and Ar² permit oxidation toform in sequence, (MgAr¹⁽⁺⁾, others neutral), (MgAr¹⁽⁺⁾, ZnAr¹⁽⁺⁾, withMgAr² and ZnAr² neutral], and so forth until two holes have been removedfrom all of the four metalloporphyrins, i.e., [MgAr¹⁽⁺⁺⁾, ZnAr¹⁽⁺⁺⁾,MgAr²⁽⁺⁺⁾, ZnAr²⁽⁺⁺⁾]. Thus, up to eight holes can be stored in thememory with each unique oxidation state serving as a digit of a basiceight-bit memory element. This is illustrated below in Table 1.

TABLE 1 Bit architecture in a prototype SHMU storage molecule. SubunitP1 Subunit P2 Subunit P3 Subunit P4 Memory MgAr¹ ZnAr¹ MgAr² ZnAr²“parity” 0 0 0 0 000 + 0 0 0 001 + + 0 0 010 ++ + 0 0 011 ++ ++ 0 0 100++ ++ + 0 101 ++ ++ + + 110 ++ ++ ++ + 111 ++ ++ ++ ++

The synthetic methodologies already established permit the extension ofthe linear architecture, thus increasing the dynamic range of the basicmemory element well beyond the three bits indicated. Conversely, themolecule could be reduced to two subunits thereby encoding 2 bits (+“parity”). In addition, subunits can be engineered that have more thantwo oxidation states. Thus for example, molecules and/or subunits can beengineered that have virtually any number (e.g., 2, 4, 8, 16, 32, 64,128, etc.) of different and distinguishable oxidation states.

In other embodiments, single molecule, non-polymeric molecules canmaintain multiple oxidation states and thereby support multiple bits. Inpreferred embodiments, such molecules comprise multiple redox-activesubunits. Certain preferred molecules have 2, 3, 5, 8, or even moredifferent and distinguishable non-neutral oxidation states. One suchmolecule is illustrated by Formula XI.

where, F is a redox-active subunit (e.g., a ferrocene, a substitutedferrocene, a metalloporphyrin, or a metallochlorin, etc.), J¹ is alinker, M is a metal (e.g., Zn, Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co,Rh, Ir, Mn, B, Al, Ga, Pb, and Sn), S¹ and S² are independently selectedfrom the group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen,alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano,thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido,amido, and carbamoyl wherein said substituents provide a redox potentialrange of less than about 2 volts, K¹, K², K³, and K⁴ are independentlyselected from the group consisting of N, O, S, Se, Te, and CH; L is alinker; X is selected from the group consisting of a substrate, areactive site that can covalently couple to a substrate, and a reactivesite that can ionically couple to a substrate. In some embodiments L-Xcan be eliminated and replaced with a substituent independently selectedfrom the same group as S¹ or S².

In preferred embodiments, the molecule has at least three different anddistinguishable oxidation states. Particularly preferred variants ofthis storage molecule are illustrated by Formulas XII, XIII, and XIV,below:

where K⁵, K⁶, K⁷, and K⁸ are independently selected from the groupconsisting of N, O, S, Se, Te, and CH; S² and S³ are independentlyselected from the group consisting of aryl, phenyl, cycloalkyl, alkyl,halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl,cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl,imido, amido, and carbamoyl wherein said substituents provide a redoxpotential range of less than about 2 volts, and M² is a metal (e.g., Zn,Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co, Rh, Ir, Mn, B, Al, Ga, Pb, andSn). These molecules can exist in three different and distinguishableoxidation states. The values of the oxidation states are determined bythe metal (M), the substituent(s) (S¹, S², and S²), and the redox-activesubunit (e.g. porphyrin, chlorin, or ferrocene).

Even more preferred embodiments include the molecules of Formulas XV,XVI, and XVII.

A molecule capable of storing even more information is illustrated inFormula XVIII.

where M is a metal (e.g., Zn, Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co,Rh, Ir, Mn, B, Al, Ga, Pb, and Sn), F¹, F², and F³ are independentlyselected ferrocenes or substituted ferrocenes, J¹, J², and J³ areindependently selected linkers, K¹, K², K³, and K⁴ are independentlyselected from the group consisting of N, O, S, Se, Te, and CH; L is alinker; and X is selected from the group consisting of a substrate, areactive site that can covalently couple to a substrate, and a reactivesite that can ionically couple to a substrate. In some embodiments, L-Xcan be eliminated and replaced with a substituent (i.e., a ferrocene, asubstituted ferrocene, aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy,alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato,nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, andcarbamoyl. Preferred substituents provide a redox potential range ofless than about 5 volts, preferably less than about 2 volts, morepreferably less than about 1 volt. In preferred embodiments, J¹, J², andJ³ are selected from the group consisting of 4,4′-diphenylethyne,4,4′-diphenylbutadiyne, 4,4′-biphenyl, 1-4-phenylene, 4,4′-stilbene,1,4-bicyclooctane, 4,4′-azobenzene, 4,4′-benzylideneaniline, and4,4″-terphenyl

In certain particularly preferred embodiments, in the molecules ofFormula XVIII, K¹, K², K³ and K⁴ are the same, M is a metal selectedfrom the group consisting of Zn, Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co,Rh, Ir, Mn, B, Pb, Al, Ga, and Sn, J², J², and J³ are the same; and F¹,F², and F³ are all different. One preferred embodiment is a 5 bitmolecule illustrated by Formula XIX.

In this example, two oxidation states are determined by the porphyrin,and the remaining three states are determined by the three ferrocenes.

Still another preferred embodiment, includes molecules represented byFormula XX:

where K¹, K², K³, and K⁴ are independently selected from the groupconsisting of N, S, O, Se, Te, and CH; M is a metal or (H,H); S¹, S²,and S³ are independently selected from the group consisting of aryl,phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl,perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino,acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; L is present orabsent and, when present, is a linker; and X is selected from the groupconsisting of a substrate, a reactive site that can covalently couple toa substrate, and a reactive site that can ionically couple to asubstrate. In some embodiments L-X can be eliminated and replaced with asubstituent independently selected from the same group as S¹ or S².Preferred substituents (S¹, S², or S³) provide a redox potential rangeof less than about 2 volts. In some preferred variants M is Zn, Mg, Cd,Hg, Cu, Ag, Au, Ni, Pd, Pt, Co, Rh, Ir, Mn, B, Al, Pb, Ga, or Sn. Morepreferably M is Zn, Mg, or (H,H). In some preferred variants, S ismesityl, C₆F₅, 2,4,6-trimethoxyphenyl, or n-pentyl. In some preferredvariants, S¹, S², and S³ are independently CONH(Et), COCH₃, or H. Insome particularly preferred variants, L-X is absent or present, and whenpresent, L-X is 4-(2-(4-mercaptophenyl)ethynyl)phenyl,4-mercaptomethylphenyl, 4-hydroselenophenyl,4-(2-(4-hydroselenophenyl)ethynyl)phenyl, 4-hydrotellurophenyl, or4-(2-(4-hydrotellurophenyl)ethynyl)phenyl.

In some more preferred embodiments of Formula XX, S¹, S², and S³ are allthe same, K¹, K², K³, and K⁴ are all N; and L is p-thiophenyl. M is thenpreferably Zn or (H,H). Particularly preferred variants are listed inTable 2.

TABLE 2 Preferred variants of Formula XX. Variant S¹ and/or S² and/or S³X M 1 Mesityl SCONH(Et) H, H 2 Mesityl SCONH(Et) Zn 3 Mesityl SCOCH₃ H,H 4 Mesityl SCOCH₃ Zn 5 Mesityl SH Zn 6 C₆F₅ SCONH(Et) H, H 7 C₆F₅ SH Zn8 2,4,6-trimethoxyphenyl SCONH(Et) H, H 9 2,4,6-trimethoxyphenylSCONH(Et) Zn 10 n-pentyl SCONH(Et) H, H 11 n-pentyl SH ZnIn particularly preferred variants of the compounds indicated in Table3, L can be a phenyl.

Other preferred molecules are illustrated by Formula XXI:

where K¹, K², K³, and K⁴ are independently selected from the groupconsisting of N, O, S, Se, Te, and CH; M is a metal or (H,H); L¹, L²,and L³, and L⁴ are independently present or absent and, when present,are linkers; and X¹, X², X³, and X⁴ are independently selected from thegroup consisting of a substrate, a reactive site that can covalentlycouple to a substrate, and a reactive site that can ionically couple toa substrate. In some embodiments L-X can be eliminated and/or replacedwith a substituent independently selected from various substituents suchas aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio,perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro,amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, andcarbamoyl. Preferred substituents provide a redox potential range ofless than about 5 volts, preferably less than about 2 volts, morepreferably less than about 1 volt.

In preferred embodiments, M is Zn, Mg, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt,Co, Rh, Ir, Mn, B, Pb, Al, Ga, or Sn and in some embodiments, M is morepreferably Zn, Mg, or (H,H). In certain preferred embodiments, L¹-X¹,L²-X², L³-X³, and L⁴-X⁴ are independently present or absent and, whenpresent, are independently 3-mercaptophenyl, 3-mercaptomethylphenyl,3-(2-(4-mercaptophenyl)ethynyl)phenyl,3-(2-(3-mercaptomethylphenyl)ethynyl)phenyl, 3-hydroselenophenyl,3-hydroselenomethylphenyl, 3-(2-(4-hydroselenophenyl)ethynyl)phenyl,3-(2-(3-hydroselenophenyl)ethynyl)phenyl, 3-hydrotellurophenyl,3-hydrotelluromethylphenyl and3-(2-(4-hydrotellurophenyl)ethynyl)phenyl, or3-(2-(3-hydrotellurophenyl)ethynyl)phenyl

Particularly preferred variants of Formula XXI are illustrated by thecompounds of Formulas XXII, XXIII, and XXIV:

Using the examples and teaching provided herein, one of skill canproduce a virtually limitless supply of data storage molecules suitablefor use in the SHMU storage format of the apparatus of this invention.

C) Dynamic Hole Multi-unit (DHMU) Storage.

In another embodiment, the data storage medium used in the devices ofthis invention includes one or more molecules that act as a dynamicmulti-unit (DHMU) molecular memory storage. In one embodiment, such astorage molecule comprises a porphyrinic macrocycle containing at leasttwo porphyrins of equal energies held apart from each other at a spacingless than about 50 Å such that said molecule has an odd hole oxidationstate permitting the hole to hop between said two porphyrins and whereinsaid odd hole oxidation state is different from and distinguishable fromanother oxidation state of said porphyrinic macrocycle.

The basic unit of a dynamic hole multi-unit storage molecule isillustrated by Formula XXV.P¹—P²—P³  XXV.where P² is a redox-active subunit having an oxidation potential higherthan P¹ or P³ and P¹ and P³ have the essentially the same oxidationpotential. Thus, when an electron is withdrawn from the molecule, the“hole” does not reside on P¹ and, instead, “hops” from P¹ to P³ and backagain. Data are stored in the “hopping” hole. As will be explainedbelow, this permits interrogation of the molecule without resetting thestate of the molecule. Accordingly, a “read” can be performed without a“refresh”.

One particularly preferred DHMU storage molecule is illustrated byFormula XXVI:

where P¹, P³, P⁴, and P⁶ are independently selected porphyrinicmacrocycles; J¹, J², J³, and J⁴ are independently selected linkers thatpermit electron transfer between the porphyrinic macrocycles; P² and P⁵are independently selected metallo-free porphyrinic macrocycles; and Qis a linker. Preferred “Q” linkers include, but are not limited tolinkers such as 1,4-bis(4-terphen-4″-yl)butadiyne or atetrakis(arylethyne), or linkers comprised of 1,12-carboranyl(C₂B₁₀H₁₂), 1,10-carboranyl (C₂B₈H₁₀), [n]staffane, 1,4-cubanediyl,1,4-bicyclo[2.2.2]octanediyl, phenylethynyl, or p-phenylene units.

One particularly preferred variant of this molecule is illustrated inFormula XVII.

where M¹ and M² are independently selected metals; S¹, S², S³, S⁴, S⁵,S⁶, S⁷, and S⁸ are independently selected from the group consisting ofaryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio,perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro,amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, andcarbamoyl; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are independently selectedfrom the group consisting of are independently selected from the groupconsisting of N, O, S, Se, Te, and CH; L¹ and L² are independentlyselected linkers; and X¹ and X² are independently selected from thegroup consisting of a substrate, a reactive site that can covalentlycouple to a substrate, and a reactive site that can ionically couple toa substrate. Preferred substituents (S¹, S², S³, S⁴, S⁵, S⁶, S⁷ or S⁸)provide a redox potential range of less than about 5 volts, morepreferably less than about 2 volts, and most preferably less than about1 volt. In some embodiments L-X can be eliminated and replaced with asubstituent independently selected from the same group as S¹–S⁸.

In particularly preferred DHMU storage molecules of Formula XVII, S¹,S², S³, S⁵, S⁶, S⁷, are the same, S⁴ and S⁸ are the same; K¹, K², K³,K⁴, K⁵, K⁶, K⁷, and K⁸ are the same, J¹, J², J³ and J⁴ are the same; andM¹ and M² are different. A preferred species is illustrated by FormulaXXVIII:

The overall architecture of these molecule consists of linear trimers(e.g. like Formula XXV) joined together by a linker (e.g., a1,4-bis(4-terphen-4″-yl)butadiyne or a tetrakis(arylethyne) unit). Insome preferred embodiments, trimers consist of metallo-free base-metalloporphyrins (see, e.g., Formula XVIII).

In preferred embodiments, the peripheral porphyrins in a given trimerhave identical metals and substituents engendering equivalent redoxpotentials. The core free base porphyrins each have perfluorophenylsubstituents to render the porphyrins more resistant to oxidation. Thecentral linker (e.g., a 1,4-bis(4-terphen-4″-yl)butadiyne or atetrakis(arylethyne)) serves as a structural unit to hold the trimerstogether. In addition, each porphyrin bears a linker (e.g., ap-thiophenol unit) for assembly on electroactive surfaces. Thisnanostructure, although complex in appearance, is in fact substantiallysmaller than other nanostructures synthesized and known in the priorart.

Information is stored in the dynamic hole memory via oxidation of theporphyrinic macrocycles as described above for the static-hole memory.However, there are certain key differences that distinguish the twotypes of memory elements that are illustrated by reference to FormulaXVIII. In compounds of Formula XVIII, the oxidation potentials of thetwo Mg porphyrins are essentially identical to one another (thedifference is less than thermal energy at room temperature), as is alsothe case for the two Zn porphyrins. Thus, oxidation results in thefollowing sequence of states: [MgP₁ ⁺, others neutral], [MgP₁ ⁺, MgP₂ ⁺,both ZnP₃ and ZnP₄ neutral], [MgP₁ ⁺, MgP₂ ⁺, ZnP₃ ⁺, ZnP₄], [MgP₁ ⁺,MgP₂ ⁺, ZnP₃ ⁺, ZnP₄ ⁺], [MgP₁ ⁺⁺, MgP₂ ⁺, ZnP₃ ⁺, ZnP₄ ⁺], and so forthuntil two holes have been removed from each metalloporphyrin, i.e. [MgP₁⁺⁺, MgP₂ ⁺⁺, ZnP₃ ⁺⁺, ZnP₄ ⁺⁺]. Thus, up to eight holes can again bestored in the nanostructure.

However, the cases where one hole (or three holes) resides on either theMg or the Zn porphyrins are unique. For these odd-hole oxidation states,the hole(s) rapidly hop between the two metalloporphyrins (100's of KHzto 100's of MHz, depending on the type of porphyrin. In contrast, wheneach Mg or Zn porphyrin contains the same number of holes, no hoppingcan occur.

In a preferred embodiment, information is stored only via thehole-hopping states of the multiporphyrin nanostructure, hence thedesignation “dynamic-hole” multi-unit storage. The encoding of aprototypical DHMU storage cell using the hole-hopping states is shown inFIG. 5 (the double-headed arrows indicate hole hopping). The syntheticmethodologies already established permit extension of the architecturevia addition of other trimeric units wherein the oxidation potential ofthe metalloporphyrin is different from that of the others, thusincreasing the dynamic range of the basic memory element beyond thatshown.

V. Synthesis and Characterization of Storage Medium Molecule(s).

A) Designing Oxidation States into the Storage Medium Molecule(s).

Control over the hole-storage and hole-hopping properties of theredox-active units of the storage molecules used in the memory devicesof this invention allows fine control over the architecture of thememory device.

Such control is exercised through synthetic design. The hole-storageproperties depend on the oxidation potential of the redox-active unitsor subunits that are themselves or are that are used to assemble thestorage media used in the devices of this invention. The hole-storageproperties and redox potential can be tuned with precision by choice ofbase molecule(s), associated metals and peripheral substituents (Yang etal. (1999) J. Porphyrins Phthalocyanines, 3: 117–147).

For example, in the case of porphyrins, Mg porphyrins are more easilyoxidized than Zn porphyrins, and electron withdrawing or electronreleasing aryl groups can modulate the oxidation properties inpredictable ways. Hole-hopping occurs among isoenergetic porphyrins in ananostructure and is mediated via the covalent linker joining theporphyrins (Seth et al. (1994) J. Am. Chem. Soc., 116: 10578–10592, Sethet al (1996) J. Am. Chem. Soc., 118: 11194–11207, Strachan et al. (1997)J. Am. Chem. Soc., 119: 11191–11201; Li et al. (1997) J. Mater. Chem.,7: 1245–1262, Strachan et al. (1998) Inorg. Chem., 37: 1191–1201, Yanget al. (1999) J. Am. Chem. Soc., 121: 4008–4018). Hole-hopping is notexpected in the SHMU storage molecule(s) because each porphyrin has adifferent oxidation potential. Hole-hopping is expected amongisoenergetic porphyrins in the DHMU molecule(s).

We have studied hole-hopping phenomena extensively in relatednanostructures in solution. We also have prepared and characterized theelectrochemical properties of a library of monomeric Mg or Zn porphyrinsbearing diverse aryl groups (Yang et al. (1999) J. PorphyrinsPhthalocyanines, 3: 117–147). The effects of metals on metalloprophyrinoxidation potentials are well known (Fuhrhop and Mauzerall (1969) J. Am.Chem. Soc., 91: 4174–4181). Together, these provide a strong foundationfor designing devices with predictable hole-storage and hole-hoppingproperties.

The design of compounds with predicted redox potentials is well known tothose of ordinary skill in the art. In general, the oxidation potentialsof redox-active units or subunits are well known to those of skill inthe art and can be looked up (see, e.g., Handbook of Electrochemistry ofthe Elements). Moreover, in general, the effects of various substituentson the redox potentials of a molecule are generally additive. Thus, atheoretical oxidation potential can be readily predicted for anypotential data storage molecule. The actual oxidation potential,particularly the oxidation potential of the information storagemolecule(s) or the information storage medium can be measured accordingto standard methods. Typically the oxidation potential is predicted bycomparison of the experimentally determined oxidation potential of abase molecule and that of a base molecule bearing one substituent inorder to determine the shift in potential due to that particularsubstituent. The sum of such substituent-dependent potential shifts forthe respective substituents then gives the predicted oxidationpotential.

B) Synthesis of Storage Medium Molecules.

The basic synthetic methodologies used to construct the storage mediummolecules of this invention are described in Prathapan et al. (1993) J.Am. Chem. Soc., 115: 7519–7520, Wagner et al. (1995) J. Org. Chem., 60:5266–5273, Nishino et al. (1996) J. Org. Chem., 61: 7534–7544, Wagner etal. (1996) J. Am. Chem. Soc., 118: 11166–11180, Strachan et al. (1997)J. Am. Chem. Soc., 119: 11191–11201, and Li et al. (1997) J. Mater.Chem., 7: 1245–1262. These papers describe various strategies for thesynthesis of a number of multi-porphyrin (porphyrinic macrocycle)compounds. More particularly, these papers which focus on light capture,energy funneling, and optical gating, has led to the preparation ofnanostructures containing up to 21 covalently linked porphyrins (Fenyoet al. (1997) J. Porphyrins Phthalocyanines, 1: 93–99, Mongin et al.(1998) J. Org. Chem., 63: 5568–5580, Burrell and Officer (1998) Synlett1297–1307, Mak et al. (1998) Angew. Chem. Int. Ed. 37: 3020–3023, Nakanoet al. (1998) Angew. Chem. Int. Ed. 37: 3023–3027, Mak et al. (1999)Chem. Commun., 1085–1086). Two-dimensional architectures, such asmolecular squares (Wagner et al. (1998) J. Org. Chem., 63: 5042–5049),T-shapes (Johnson, T. E. (1995), Ph.D. Thesis, Carnegie MellonUniversity), and starbursts (Li et al. (1997) J. Mater. Chem., 7:1245–1262) all comprised of different covalently linked porphyrinconstituents, have also been prepared.

In addition, the hole storage and dynamic hole mobility characteristicsof the multiporphyrin nanostructures have been investigated in detailduring the course of our other studies of these materials (Seth et al.(1994) J. Am. Chem. Soc., 116: 10578–10592, Seth et al (1996) J. Am.Chem. Soc., 118: 11194–11207, Strachan et al. (1997) J. Am. Chem. Soc.,119: 11191–11201; Li et al. (1997) J. Mater. Chem., 7: 1245–1262,Strachan et al. (1998) Inorg. Chem., 37: 1191–1201, Yang et al. (1999)J. Am. Chem. Soc., 121: 4008–4018).

The general synthetic strategy involves the following approaches: (1) Amodular building block synthesis of covalent multiporphyrinnanostructures; and (2) The directed self-assembly of the resultingnanostructures on electrode (e.g. gold electrode) surfaces.

The methods for synthesis, purification, and characterization for themolecular memory molecules (MMMs) generally follow those employed in themodular stepwise synthesis (Lindsey et al. (1994) Tetrahedron, 50:8941–8968) of molecular wires (Wagner et al. (1994) J. Am. Chem. Soc.,116: 9759–9760), optoelectronic gates (Wagner et al. (1996) J. Am. Chem.Soc., 118: 3996–3997) and light-harvesting nanostructures (Prathapan etal. (1993) J. Am. Chem. Soc., 115: 7519–7520, Johnson, T. E. (1995),Ph.D. Thesis, Carnegie Mellon University, Wagner et al. (1996) J. Am.Chem. Soc., 118: 11166–11180, Li et al. (1997) J. Mater. Chem., 7:1245–1262, and Li et al. (1998) J. Am. Chem. Soc., 120: 10001–10017). Inpreferred embodiments, the following synthetic methods form thefoundation for the building block synthesis of multiporphyrinnanostructures:

-   (1) A room temperature one-flask synthesis of meso-substituted    porphyrins (Lindsey et al. (1987) J. Org. Chem. 52: 827–836, Lindsey    et al. (1994) J. Org. Chem. 59: 579–587, Li et al. (1997)    Tetrahedron, 53: 12339–12360).-   (2) Incorporation of bulky groups around the porphyrin to achieve    enhanced solubility in organic solvents (Lindsey and    Wagner (1989) J. Org. Chem., 54: 828–836).-   (3) A one-flask synthesis of dipyrromethanes, key building blocks in    the synthesis of porphyrins bearing 2–4 different meso-substituents    (Lee and Lindsey (1994) Tetrahedron, 50: 11427–11440, Littler et    al. (1999) J. Org. Chem., 64: 1391–1396).-   (4) A synthesis of trans-substituted porphyrins without acidolytic    scrambling (Littler et al. (1999) J. Org. Chem., 64: 2864–2872).-   (5) A 9-step synthesis of porphyrins bearing 4 different    meso-substituents (Lee et al. (1995) Tetrahedron, 51: 11645–11672).-   (6) Mild methods for inserting magnesium (Lindsey and    Woodford (1995) Inorg. Chem. 34: 1063–1069, O'Shea et al. (1996)    Inorg. Chem., 35: 7325–7338) or other metals (Buchler, J. W. In The    Porphyrins; Dolphin, D. Ed.; Academic Press: New York. 1978; Vol. I,    pp. 389–483) into porphyrins.-   (7) Efficient Pd-mediated coupling reactions (60–80% yields in 1–2 h    at 35° C.) for constructing diphenylethyne linkers joining the    porphyrins (Wagner et al. (1995) J. Org. Chem., 60: 5266–5273).

In one embodiment, building blocks are synthesized using methodsdescribed by Wagner et al. (1996) J. Am. Chem. Soc., 118: 11166–11180,Strachan et al. (1997) J. Am. Chem. Soc., 119: 11191–11201, Wagner etal. (1996) J. Am. Chem. Soc., 118:3996–3997, Li et al. (1997) J. Mater.Chem., 7: 1245–1262; Lindsey et al. (1994) Tetrahedron, 50: 8941–8968;Wagner et al. (1994) J. Am. Chem. Soc., 116: 9759–9760; Lindsey andWagner (1989) J. Org. Chem., 54: 828–836; Lee and Lindsey (1994)Tetrahedron, 50: 11427–11440; Lee et al. (1995) Tetrahedron, 51:11645–11672; Lindsey and Woodford (1995) Inorg. Chem. 34: 1063–1069; andWagner et al. (1995) J. Org. Chem., 60: 5266–5273.

The synthesis of the molecules that form the basis for the storagemolecules (e.g., SHMU storage molecules, DHMU storage molecules, etc.)is performed using a modular building block approach. This approachemploys a stepwise synthesis (rather than polymerization) and yieldshighly purified and well-characterized products. One approach, utilizesa series of redox-active “building blocks” (e.g., a series of monomericporphyrinic macrocycles or ferrocene constituents) that can be linked tothe gold substrate that will serve as one of the electrodes in the chip.Preferred monomeric redox-active units that are prepared have differentoxidation potentials that fall in the range from 0 to 1.3 volts.

The two different redox-active units can be linked together to form abasic dimeric architecture. Similarly, two other different redox-activeunits (e.g. porphyrins) can be linked to form a second dimericarchitecture. Then the two dimers can be linked to form a linear, ornon-linear, tetrameric architecture consisting of four different typesof redox-active units (e.g., porphyrins).

One example of a preferred synthesis approach is shown in FIG. 6. Thereaction of 5-mesityldipyrromethane (Lee and Lindsey (1994) Tetrahedron,50: 11427–11440) with two aldehydes affords three porphyrins, includingthe desired mono-thiol porphyrin. The latter is metalated with Zn or Mg,and then the thiol protecting group can be removed. Of the various thiolprotecting groups (Hsung et al. (1995) Tetrahedron Lett., 36: 4525–4528;Ricci et al. (1977) J. Chem. Soc. Perkin Transactions I., 1069–1073) theS-acetyl or S-(N-ethyl-carbamoyl) group is stable toward the requiredsynthetic conditions yet cleaved easily with methanolic diethylamine.The useful precursor 4-mercaptobenzaldehyde is readily available (Younget al. (1984) Tetrahedron Lett., 25: 1753–1756). The resulting porphyrinmono-thiol can be assembled on a gold surface, or the protected thiolcan be deprotected in situ on a gold surface.

The synthesis of an SHMU storage molecule is shown in FIG. 7. Fourporphyrin building blocks are employed in the synthesis of thisnanostructure. Each building block is available via establishedsynthetic routes (either via the route we established forABCD-porphyrins or via a 3+1 route involving a tripyrrane) (Lee et al.(1995) Tetrahedron, 51: 11645–11672). The fundamental methodology forjoining the porphyrin building blocks involves Pd-mediated coupling ofan ethynyl-porphyrin and an iodo-porphyrin (Wagner et al. (1995) J. Org.Chem., 60: 5266–5273). Our optimized conditions for these couplingreactions afford 60–80% yields in 2–4 h. Purification is achieved usingsize-exclusion chromatography and characterization is accomplished withlaser desorption mass spectrometry (see, e.g., Fenyo et al. (1997) J.Porphyrins Phthalocyanines, 1: 93–99). This synthetic route is toleranttoward diverse aryl groups and metals in the porphyrin unit.

The stepwise synthesis makes available the dimeric unit of the SHMUstorage molecule upon one cycle of coupling. The dimer will be examinedelectrochemically. Following cleavage of the S-acetyl protecting group,the thiols may undergo oxidative coupling (forming the disulfides)during handling and processing. Such disulfides can be reduced toregenerate the thiols, or deposited on gold surfaces whereupon reductionin situ yields the bound thiol species. Alternatively, the S-acetylgroups can be cleaved in situ upon exposure to the metal (e.g., gold)surface (Tour et al. (1995) J. Am. Chem. Soc., 117: 9529–9534). A numberof porphyrin thiols have been prepared and deposited on metals but notfor memory storage applications (see, e.g., Zak et al. (1993) Langmuir,9: 2772–2774; Hutchison et al. (1993) Langmuir 9: 3277–3283; Bradshaw etal. (1994) Gazz. Chim. Ital. 124, 159–162; Postlethwaite et al. (1995)Langmuir, 11: 4109–4116; Akiyama et al. (1996) Chem. Lett, 907–908;Uosaki et al. (1997) J. Am. Chem. Soc., 119: 8367–8368; Katz and Willner(1997) Langmuir, 13: 3364–3373; Ishida et al. (1998) Chem. Lett.,267–268; Ishida et al. (1998) Chem. Commun., 57–58). Nanostructureshaving up to 21 porphyrins are readily synthesized.

A representative synthesis of a DHMU storage molecule is shown in FIG.8. Three porphyrin building blocks are prepared and metalated withmagnesium or zinc. The synthetic strategy builds the two arms of theDHMU storage molecule separately, which are then coupled in thepenultimate step of the synthesis. Each arm is constructed via twoPd-mediated couplings, yielding the respective trimers. One trimer isiodinated at the ethyne (Barluenga et al. (1987) Synthesis, 661–662; andBrunel and Rousseau (1995) Tetrahedron Lett., 36: 2619–2622) then joinedwith the other trimer in a heterocoupling process to form the H-likestructure. A variety of conditions can be employed for theheterocoupling reaction (Alami and Ferri (1996) Tetrahedron Lett., 37:2763–2766). We previously showed that Pd-mediated (copper-free)couplings can be employed for homocoupling reactions (Wagner et al.(1995) J. Org. Chem., 60: 5266–5273). Copper-free couplings arepreferred to avoid copper insertion in the free base porphyrin. Here thesame Pd-mediated coupling is used to perform the heterocoupling. Thefinal step is cleavage of the S-acetyl protecting group, which proceedsin methanolic Et₂NH. Such conditions do not alter any of the otherfunctionalities in the molecule. Alternatively, the S-acetyl groups canbe cleaved in situ upon exposure to the metal (e.g., gold) surface (Touret al. (1995) J. Am. Chem. Soc., 117: 9529–9534).

Using the synthesis strategies exemplified here and in the Examples, oneof ordinary skill in the art can routinely produce relatively complexdata storage molecules for use in the devices of this invention.

C) Characterization of the Storage Media.

The storage media molecule(s), once prepared, can be characterizedaccording to standard methods well known to those of skill in the art.The characterization of multiporphyrin nanostructures has been described(see, e.g., Strachan et al. (1997) J. Am. Chem. Soc., 119: 11191–11201;Wagner et al. (1996) J. Am. Chem. Soc., 118: 3996–3997; Li et al. (1997)J. Mater. Chem., 7: 1245–1262; Seth et al. (1996) J. Am. Chem. Soc.,118: 11194–11207; Seth et al. (1994) J. Am. Chem. Soc., 116:10578–10592). In a preferred embodiment, the electrochemical studiesinclude cyclic and square-wave voltammetry to establish the redoxpotentials of the monomeric and multi-unit constituents of the storagemedia. Bulk electrochemical oxidations are performed on each of thestorage materials to assess the hole-storage capabilities and thestability. Absorption and vibrational spectroscopic methods are used toassess the structural and electronic properties of both the neutral andoxidized materials. Electron paramagnetic resonance techniques are usedto probe the hole-storage and hole-mobility characteristics of theoxidized storage molecules. Using the above-identified techniques,benchmarks for the expected performance characteristics of a storagemolecule (e.g., oxidation potentials, redox reversibility, dynamichole-mobility characteristics, etc.) can be ascertained.

D) Self-assembly of the Storage Medium Molecules on Target Substrates.

In preferred embodiments, the storage molecules comprising the storagemedium are designed to self-assemble on a substrate (e.g. a metal suchas gold). The disk-like structure of the porphyrin macrocycles engendersself-assembly. Self-assembled monolayers of porphyrins on solidsubstrates are well known and have been extensively studied (see, e.g.,Schick et al. (1989) J. Am. Chem. Soc., 111: 1344–1350, Mohwald et al.(1986) Thin Solid Films, 141: 261–275).

To exert control over the pattern of self-assembly, reactive sites (e.g.thiols) or linkers bearing active sites are incorporated into thestorage molecules (nanostructures). The reactive sites bind to thetarget (e.g. gold electrode) surface giving an organized self-assembledstructure. In the case of porphyrins with thiol linkers attached to themeso-positions, the porphyrins arrange in upright orientations.Non-covalent interactions between storage molecules are typically weak,particularly when bulky aryl groups are attached to each of theporphyrins.

VI. Writing to the Storage Device.

In preferred embodiments of the data storage devices of this invention,information is written to a particular memory location via applicationof a potential of the requisite value and temporal duration at theappropriate working and reference electrode(s) to achieve the desireddigital value. The information can be erased via application of apotential of the opposite sign.

The writing process is illustrated with respect to storage of data in astatic hole multi-unit storage molecule (SHMU storage molecule). Oneparticular such molecular memory is illustrated by Formula IX and thewriting process is summarized below in Table 3.

As shown in Table 3, each porphyrin has two redox processes, each ofwhich is separated by at least 150 mV. To activate bit 001, a potentialgreater than 0.38 V (but less than 0.51 V) would be applied to thememory element to oxidize the magnesium porphyrin to its first oxidationstate. The other porphyrins in the SHMU storage molecule could then besequentially oxidized through the various redox states to provide thedifferent bits. In preferred embodiments, this is accomplished throughthe application of very short (e.g., microsecond) pulses applied at avoltage sufficient to oxidize a porphyrin to the appropriate redoxstate. This process is summarized in FIG. 9. Thus, each redox state ofthe composite porphyrinic nanostructure can independently accessed toprovide one bit of resolution. This can be accomplished via theelectrochemical oxidation of the molecule in stepwise increments.

TABLE 3 Redox properties of model metalloporphyrins (MP). Bit Redoxprocess E⁰ (V vs. Ag/AgCl) 000 All redox components in neutral state 001MgPZnP → ← MgP⁺ZnP + 1 e⁻ 0.38 010 MgP⁺ZnP → ← MgP⁺ZnP⁺ + 1 e⁻ 0.51 011MgP⁺ZnP⁺ → ← MgP²⁺ZnP⁺ + 1 e⁻ 0.71

There is a great advantage to the small size of each memory element,which is essentially a modified electrode surface. When each memoryelement is reduced to sub-micron dimensions, the area of the surfaceallows the presence of only a few hundred data storage (e.g., porphyrin)molecules. Using Faraday's law, Q=nFN (where Q equals the total charge,n equals the number of electrons per molecule, F is 96,485 Coulombs/moleand N is the number of moles of electroactive species present), it canbe determined that only a small charge (1.6×10⁻¹⁶ C; if passed in 1 μs,would result in a current of roughly 160 pA) must pass in order tochange the electrochemical charge corresponding to each bit.

Additionally, the intrinsic limitation to the speed of mostelectrochemical experiments lies in the time required to charge theelectrode to the appropriate potential (the charging current, which hasa time dependence of exp(−t/RC)). Since the capacitance of the electrodeis directly proportional to its area, miniaturization of each element ofthe system to submicron dimensions will greatly increase its speed. Forexample, a square gold electrode with 0.1 μm dimensions would have acapacitance of approximately 2×10⁻¹⁹F, leading to an RC time constant ofonly 2 picoseconds. For this reason, electrode charging currents shouldbe insignificant in determining the ultimate performance of thesedevices.

The voltage used to write the data can be derived from any of a widevariety of sources. In a simple embodiment, the voltage can simply bethe output from a power supply. However, in preferred embodiments, thevoltage will be the output from some element of an electronic circuit.The voltage can be a signal, the representation of a logic state, theoutput from a gate, from an optical transducer, from a centralprocessing unit, and the like. In short, virtually any voltage sourcethat can be electrically coupled to the devices of this invention can beused to write data to the storage media therein.

VII. Reading from the Storage Device.

The storage device(s) of this invention can be read according to any ofa wide variety of methods well known to those of ordinary skill in theart. Essentially any method of detecting the oxidation state of acompound can be utilized in the methods of this invention. However,where the readout is destructive of the state of the memory cell(s)(e.g. in certain SHSU or SHMU memories), the read will preferably befollowed by a refresh to reset the oxidation state of the storage cell.

In particularly preferred embodiments, the storage medium 102 of astorage cell 100 is set to neutral (e.g., 0 potential for the system,but which might not be at true zero voltage with respect to ground)using the working electrode. The oxidation state of the memory cell isthen set by changing the potential at the reference electrode 103 (e.g.by setting the reference electrode negative to the desired voltage). Theoxidation state of the storage cell is then measured (e.g. usingsinusoidal voltammetry) via the working electrode 101. In this preferredformat, the oxidation state is assayed by measuring current. Bymeasuring current at the working electrode 101 and setting the statewith the reference electrode 103, the measurement is not made at theplace the potential is applied. This makes it far simpler todiscriminate the oxidation state. If the potential were applied to theelectrode through which the current was measured unnecessary noise wouldbe introduced into the system.

A) Reading from Static Hole Storage Media

In the case of static hole storage media (e.g. SHSU and SHMU), thereading of information from a particular memory location is achievedextremely rapidly by sweeping a potential over the full range used toestablish the dynamic range of the storage element. The fidelity of themeasurement is dependent on how well the oxidation state of theindividual storage element can be determined. Traditionally,electrochemical methods could only improve the signal to noise ratio bydiscriminating the faradaic signal from the background components in thetime domain through application of pulse waveforms (i.e., differentialpulse polarography, square wave voltammetry). These methods discriminatethe faradaic current from the charging current in the time domain, sincecharging currents decay much more rapidly than the faradaic current(exp(−t/RC) vs t^(−1/2), respectively). However, the analytical faradaiccurrent is not totally discriminated from the charging current, and mostof the signal is discarded because sampling is done late in the pulsecycle.

More recently, sinusoidal voltammetry (SV) has been shown to havesignificant advantages over traditional waveforms in an electrochemicalexperiment (Singhal et al. (1997) Anal. Chem., 69: 1662–1668. Forexample, the background current resulting from cyclic voltammetry(consisting primarily of charging current) resembles a square wave,which contains significant intensity at both fundamental and oddharmonic frequencies. In contrast, the charging current resulting fromsine wave excitation has only one frequency component centered at thefundamental, while the faradaic current is distributed over manyfrequencies as is illustrated in FIG. 10. This characteristic of sinewave excitation simplifies the electroanalytical measurement, since thesignal from each oxidation state can be fine-tuned by “locking-in” onone of the higher frequency harmonics. Ultimately, the speed at whichthis can be performed is only limited by the kinetics of the redoxreaction, which may ultimately lead to megahertz frequency operation.

Since most electrochemical methods rely on differences between theE_(1/2)'s (E_(1/2) is the potential at which half of the subjectmolecules are oxidized or reduced to a particular oxidation state) todifferentiate compounds present in a sample and thereby to generate theselectivity for the measurement, this has severely limited the utilityof electrochemical methods for the analysis of many complex matrices. Incontrast, sinusoidal voltammetry can exploit the vast diversity inelectron transfer rates observable at solid electrodes (k⁰, the rate ofelectron transfer) can vary over ten orders of magnitude at the sameelectrode surface) to obtain additional selectivity in theelectrochemical measurement.

The composition of the frequency spectrum is extremely dependent on therate of electron transfer. By adjusting the frequency of the sinusoidal(or other time-varying) excitation waveform, it becomes possible to usethis kinetic information as well as the phase information todiscriminate between two molecules which have very similarelectrochemical properties. For example, this technique has been usedfor the detection of the direct oxidation of double-stranded DNA atcopper electrodes (Singhal and Kuhr (1997) Anal. Chem., 69: 1662–1668).Where this is usually undetectable at conventional electrodes withstandard voltammetric techniques, the use of sinusoidal voltammetryallowed the measurement of 1.0 nM double-stranded DNA. The concentrationdetection limit (S/N=3) for this size of dsDNA at the 6th harmonic is3.2 pM. When coupled with a low-volume system, such as a monolayer ofthe adsorbed material, this allows detection of sub-zeptomole (10⁻²¹mole) quantities of the storage medium molecule(s) on the surface.

This procedure may ultimately degrade the memory in the absence of arefresh mechanism. The level of degradation will depend on the totalnumber of molecules ultimately used to ensure acceptable faulttolerance. To avoid degradation problems, however, a refresh cycle (awrite cycle resetting the memory to the read value) can be insertedimmediately after each read cycle is complete.

B) Reading from a Dynamic Hole Storage Medium

The same methods as described above for the static hole storage mediacan also be used to read dynamic hole storage media. However, thedynamic hole storage media were designed for and afford the uniquepossibility of interrogating a particular memory location viaexamination of the impedance of the working electrode. This readingscheme is possible because the impedance is modulated by the hole thathops between the two identical porphyrinic macrocycle units orthogonalto the surface of the electrode. The frequency of hole hopping isdifferent depending on which pair of redox-active subunits is in anodd-hole state (and whether they are in a three-hole or one-hole state).The value of the bit can be read via determination of that frequency.This is most easily accomplished by an impedance measurement (preferablya function of frequency).

The “hole-hopping” state of the porphyrin will determine the conductivestate of the molecular monolayer. Since the hole(s) rapidly hop betweenthe two metalloporphyrins in the odd-hole oxidation states at rateswhich vary from the 100's of KHz to 100's of MHz, depending on the typeof porphyrin, it is possible to find these states by the frequency atwhich the impedance of the nanostructure dips. In contrast, when each Mgor Zn porphyrin contains the same number of holes, no hopping can occur.The rate of hole hopping will determine the impedance characteristics ofeach state of each porphyrin nanostructure in the chip, and a decreasein the cell impedance would be expected at the hole-hopping frequencyfor each state of each porphyrin. While characterization of thesehole-hopping states requires the collection of the entire frequencyspectrum, the actual read cycle of the DHMU storage medium need onlymonitor a single frequency at a time. Impedance measurements usinglock-in based systems apply only one frequency at a time to theelectrode; any other frequencies are nearly totally suppressed by thelock-in amplifier. Thus, it is possible to monitor the frequencycharacteristic of hole-hopping level of each state and simultaneouslydetermine the logic level of each element in the array using lock-intechniques.

This method of reading is extremely sensitive for molecular memoriesthat utilize relatively small numbers of redox-active units (e.g.porphyrinic macrocycle nanostructures). The examination of the impedancecan also be performed without compromising the integrity of a particularmemory element.

For all I/O operations with the molecular memories of this invention,the use of molecular electronic components as on-chip buffering anddecoding circuitry is desirable although not required. Hybrid systemscan easily be produced incorporating the devices of this invention intoconventional integrated circuit packages that contain all the circuitryrequired for encoding/decoding data, reading and writing to the storageelement, monitoring fault tolerance and dynamically optimizing/selectingactive storage elements to maximize fault tolerance.

C) Instrumentation for Reading/Writing Molecular Memories.

As indicated above, the molecular memory devices can be read by any of awide variety of electrochemical technologies including amperometricmethods (e.g. chronoamperometry), coulometric methods (e.g.chronocoulometry), voltammetric methods (e.g., linear sweep voltammetry,cyclic voltammetry, pulse voltammetries, sinusoidal voltammetry, etc.),any of a variety of impedance and/or capacitance measurements, and thelike. Such readouts can be performed in the time and/or frequencydomain.

1) Fast Potentiostat/Voltammetry System.

In one preferred embodiment, readout is accomplished using a fastpotentiostat/voltammetry system. Such a system is capable of reading andwriting the memory elements, on a microsecond time scale. Such a systemcan be modified from a prototypical system described in U.S. Pat. No.5,650,061.

As illustrated in FIG. 11, a potentiostat with an RC time constant lessthan one microsecond is provided by using a fast voltage driver (e.g.,video buffer amplifier). A preferred video buffer amplifier retains ausable bandwidth beyond 20 MHz and is used to rephase the voltage andcurrent in the excitation signal to zero phase shift between voltage andcurrent. This rephasing of the excitation signal immediately before theworking electrode cancels out any phase shift which might be introducedby capacitance in the cable leading from the Arbitrary WaveformSynthesizer (AWS) function generator. An important part of the currentmonitor is a wide band op-amp. By using an op-amp with a very widegain-bandwidth product, the amplifier gain can be set to 10,000 andstill retain a bandwidth usable from DC to above 1 MHz. This allows thecollection of impedance data from electrodes as small as a 1 μm diskover a frequency range from 15 kHz to 5 MHz.

2) A Megahertz Impedance Analysis System.

An ultrafast impedance analysis system capable of characterizing theSHMU storage medium on a microsecond time scale can be constructed usingan Arbitrary Waveform Synthesizer (e.g., HP 8770A, AWS) and a 1-GHzDigitizing Oscilloscope (e.g., HP 54111D) controlled by a computersystem (e.g. HP 9000 series 300 computer system, Hewlett-Packard, PaloAlto, Calif.). The impedance data sets can be collected with the digitalscope with 8192 time domain points at 25 MHz. Thus, a full 8192 pointdata set can be acquired in a total of 328 μs. Both the excitation andthe response waveforms are measured; the excitation waveform is measuredprior to the start of the experiment so that the response acquisitionscan be done during the course of the experiment without interruption.One preferred excitation signal consists of a waveform with an amplitudeof 60 mV_((p-p)) which covers a frequency band from approximately 30 KHzto over 1 MHz. If five complete replicates of each excitation orresponse waveform are contained within the 8192 data points set capturedby the capture device (e.g. oscilloscope), because no further ensembleaveraging is needed, each full impedance spectra can be acquired in 328μs. Therefore, the whole frequency band under study can be excited andmonitored in a single acquisition. The FFT of the time domain dataprovides frequency-amplitude and frequency-phase characterization of thedata equivalent to the data given by a lock-in based system.

VIII. Use of the Storage Device in Computer Systems.

The use of the storage devices of this invention in computer systems iscontemplated. One such computer system is illustrated in FIG. 12. Thecomputer comprises a signal source (e.g. I/O device or CPU) a storagedevice of this invention and appropriate circuitry (e.g. voltammetrycircuitry) to read the state(s) of the storage device. In operation,voltages representing the bits to be stored are applied to the workingelectrodes of the storage device thereby setting the memory. Whenretrieval is necessary (e.g. for output, or further processing) thestate(s) of the storage device is read by the I/O circuitry and theinformation is passed off to other elements (e.g. CPU) in the computer.

FIG. 12 illustrates the memory devices of this invention integrated intoa standard computer architecture or computer system 200. The hardware ofsystem 200 includes a processor (CPU) 205, a memory 206 (which cancomprise molecular memory devices), a persistent storage 208 which doescomprise molecular memory devices of this invention, and hardware for agraphical user interface (GUI) 220, coupled by a local bus or interface210. The persistent memory 208 can include the elements shown in FIG.11. System 200 can further include additional hardware components (notshown).

System 200 can be, for example, a personal computer or workstation.Processor 205 can be, for example, a microprocessor, such as the 80386,80486 or Pentium™ microprocessor, made by Intel Corp. (Santa Clara,Calif.). Memory 206 can include, for example, random-access memory(RAM), read-only memory (ROM), virtual memory, molecular memory (FIG.11) or any other working storage medium or media accessible by processor205. Persistent storage 208 can include a hard disk, a floppy disk, anoptical or magneto-optical disk, a molecular memory or any otherpersistent storage medium. GUI 220 facilitates communications between auser and system 200. Its hardware includes a visual display 221 and aselector device (mouse, keyboard, etc.) 222. Through visual display 221,system 200 can deliver graphical and textual output to the user. Fromselector device 222, system 200 can receive inputs indicating the user'sselection of particular windows, menus, and menu items. Visual display221 can include, for example, a cathode-ray tube (CRT) or flat-paneldisplay screen, or a head-mounted display such as a virtual realitydisplay. Selector device 222 can be, for example, a two-dimensionalpointing device such as a mouse, a trackball, a track pad, a stylus, ajoystick, or the like. Alternatively or additionally, selector device222 can include a keyboard, such as an alphanumeric keyboard withfunction and cursor-control keys.

The software of system 200 includes an operating system 250 and anapplication program 260. The software of system 200 can further includeadditional application programs (not shown). Operating system 150 canbe, for example, the Microsoft® Windows™ 95 operating system for IBM PCand compatible computers having or emulating Intel 80386, 80486, orPentium™ processors. Alternatively, the operating system can bespecialized for operation utilizing molecular memory elements.Application program 160 is any application compatible with the operatingsystem and system 200 architecture. Persons of skill in the art willappreciate that a wide range of hardware and software configurations cansupport the system and method of the present invention in variousspecific embodiments.

EXAMPLES

The following examples are offered to illustrate, but not to limit theclaimed invention.

Example 1 Thiol-Porphyrins for Attachment to Electroactive Surfaces asMolecular Memory Devices

I. Molecular Design

This example presents the design and synthesis of porphyrins that can beattached covalently, in defined geometries, to electroactive surfaces.For the present purposes, we consider only the surface of a goldelectrode. Three design features we sought to obtain included thefollowing: (1) The ability to attach porphyrins via a sulfide linkage tothe gold electrode surface with the porphyrins oriented vertically orhorizontally. (2) The ability to tune the porphyrin electrochemicaloxidation potential through the use of electron-withdrawing or releasingsubstituents at the periphery of the porphyrin, or the use of differentmetals in metalloporphyrins. (3) The use of thiol protecting groups thatwould cleave spontaneously on the gold surface, thereby avoiding thepotential practical problems of handling free thiols.

In order to achieve a vertical orientation of the porphyrin attached tothe gold surface, we employed A₃B meso-substituted porphyrins where theB group bears the thiol for surface attachment. The remaining three Agroups bear substituents for control of the electrochemical potential.In order to achieve a horizontal orientation of the porphyrin attachedto the gold surface, we employed porphyrins possessing two or four—CH₂SH groups in the meta position of the meso-phenyl rings. The tuningof the electrochemical potential can be achieved in a straightforwardmanner in the A₃B-porphyrins, where the A group can range fromelectron-rich substituents such as mesityl to electron-deficientsubstituents such as the pentafluorophenyl group. With thehorizontally-oriented porphyrins, the introduction of substituents totune the potential must be done without interfering with the meta-CH₂SHgroups. Accordingly, we have elected to investigate different metalswith the horizontally-positioned porphyrins. The selection of the thiolprotecting group poses extensive challenges. The protecting group ofchoice should be stable under diverse conditions, including the acidicand oxidative conditions of porphyrin formation as well as conditionsfor porphyrin metalation (generally involving mild Lewis acids, in somecases in the presence of bases). One objective is to be able toconstruct diarylethyne-linked multiporphyrin arrays, which requirePd-mediated coupling reactions. We sought thiol protecting groups thatwould meet these diverse criteria.

II. Results

Aldehydes.

Our initial synthetic strategy toward mono-thiol (A₃B) porphyrins forvertical orientation started from 4-methylthiobenzaldehyde which wehoped to convert to 4-mercaptobenzaldehyde dimethyl acetal (using thestrategy of Young et al. (Young et al. (1984) Tetrahedron Lett., 25:1753–1756)) and next to the thiol-protected dimethyl acetals. The firststep involved conversion of the aldehyde group to its dimethyl acetalunder standard conditions. The sulfide obtained was successfullyconverted to the sulfoxide in 95% yield but treatment of the sulfoxidewith TFAA led to polymerization rather than the thiol (probably becauseof cleavage of acetal and intermolecular thioacetalization). We overcamethis problem by making two improvements: (1) the dimethyl acetal wasreplaced by a more bulky acetal protecting group at an earlier stage ofthe synthesis, (2) milder conditions for the Pummerer rearrangement wereemployed (Sugihara et al. (1978) Synthesis, 881). Thus protection of thecarbonyl group with neopentyl glycol (Rondestvedt (1961) J. Org. Chem.,26: 2247–2253) followed by oxidation of the resulting acetal (1)smoothly afforded sulfoxide 2 in 86% overall yield (Scheme 1, FIG. 14).Treatment of sulfoxide 2 with TFAA in the presence of 2,6-lutidinefollowed by hydrolysis of the resulting intermediate furnished thiol 3.Compound 3 was transformed into the S-protected acetals 4, 5, 6, and 7using ethyl isocyanoacetate (Ricci et al. (1977) J. Chem. Soc. PerkinTrans. 1: 1069–1073), 2,4-dinitrofluorobenzene (Vorozhtsov et al. (1958)Z. Obs. Chim., 28: 40–44, Engl. Transl. 40–44), 9-chloromethylanthracene(Kornblum and Scott (1974) J. Am. Chem. Soc., 96: 590–591), and pivaloylchloride, respectively, in overall yields of 20–65% from the sulfoxide2. The acetal group in 4, 5, 6 and 7 was selectively hydrolyzed prior toformation of the corresponding porphyrin.

Two other S-protected p-thiobenzaldehydes were obtained as shown inScheme 2 (FIG. 15). The thiocyanato-benzaldehyde 8 was preparedaccording to a general procedure (Suzuki and Abe (1996) Synth. Commun.,26: 3413–3419) in 20% yield. All attempts to improve the yield byreplacement of DMF with 1,3-dimethyl-2-imidazolidinone, increasing thetemperature, or prolonging the reaction time were unsuccessful.S-Acetylthiobenzaldehyde 9 was prepared in a two-step one-flaskprocedure. Cleavage of the methyl group of 4-methylthiobenzaldehydeaccording to a general procedure (Tiecco et al. (1982) Synthesis,478–480) followed by trapping of the resulting anion with acetylchloride afforded the desired S-acetylthiobenzaldehyde 9 in 55% yield.

Our approach toward horizontally-oriented porphyrins required access toS-protected m-(HSCH₂)benzaldehydes. The commercially availablem-(bromomethyl)benzonitrile was reduced with DiBAl-H to thecorresponding m-(bromomethyl)benzaldehyde (Wagner et al. (1997)Tetrahedron, 53: 6755–6790) (Scheme 2, FIG. 15). Substitution of thebromine with potassium thiocyanate gave the thiocyanato-benzaldehyde 10as colorless crystals in 74% yield. By using the thiocyanate asprotecting group, incorporation and protection of the sulfur unit couldbe achieved in one step.

Porphyrins.

The A₃B-porphyrins were prepared using a two-step, one-flask roomtemperature synthesis of meso-substituted porphyrins that is compatiblewith a variety of precursor aldehydes including the ortho-disubstitutedbenzaldehydes that yield facially-encumbered porphyrins (Lindsey andWagner (1989) J. Org. Chem., 54: 828–836, Lindsey, J. S. inMetalloporphyrins-Catalyzed Oxidations; Montanari, F., Casella, L.,Eds.; Kluwer Academic Publishers: The Netherlands, 1994; pp 49–86,Lindsey et al. (1994) J. Org. Chem., 59: 579–587). A mixed-aldehydecondensation of mesitaldehyde, a thiol-protected aldehyde, and pyrroleafforded a mixture of porphyrins, from which the desired thiol-protectedA₃B-porphyrin was obtained by chromatography. The acetals 4–7 werehydrolyzed with trifluoroacetic acid and the resulting aldehydes 11–14were used directly without purification in the respective porphyrinsyntheses. Thus, aldehydes 9, 11, 12, 13 or 14 as well as commerciallyavailable 4-methylthiobenzaldehyde afforded thiol-protectedA₃B-porphyrins 20, 15, 16, 17, 18 or 19, respectively, in ˜10% yield(Scheme 3, FIG. 16). The porphyrins obtained were metalated usingZn(OAc)₂.2H₂O, affording Zn-20, Zn-15, Zn-16, Zn-17, Zn-18 or Zn-19.

Examination of the behavior of various thiol-protected zinc porphyrinsrevealed that the S-(N-ethylcarbamoyl) and S-acetyl groups easilycleaved in situ and the resulting porphyrin product bound on the goldsurface (vide infra). We decided to confirm this result by also cleavingthe S-(N-ethylcarbamoyl) group in porphyrin Zn-15 using basicconditions. Treatment of porphyrin Zn-15 with sodium methoxide followedby acidic workup gave mono-thiol porphyrin Zn-21, which wasair-sensitive and proved very difficult to purify to homogeneity (theporphyrin disulfide was also present) (Scheme 4, FIG. 17). The samereaction performed with quenching by acetyl chloride afforded theS-acetyl porphyrin Zn-20. Both Zn-15 and Zn-20 were found to bind to thegold surface identically with that of the free thiol containingporphyrin Zn-21.

We attempted to insert magnesium into porphyrins 15 and 16 using MgI₂and N,N-diisopropylethylamine in CH₂Cl₂ (Lindsey and Woodford (1995)Inorg. Chem. 34: 1063–1069). In both cases we obtained complex mixturesof porphyrins due to cleavage of the protecting groups. The resultingsalts likely contain the thiolate anion complexed with the protonateddiisopropylethylamine. All attempts at acidification caused demetalationof magnesium. Magnesium insertion occurred with5,10,15-trimesityl-20-(4-thiocyanatophenyl)porphyrin under theseconditions but the Mg-chelate could not be purified to homogeneity. Thedifficulty in purification may stem from lability of the thiocyanategroup on alumina, as we observed that the thiocyanate group of5,10,15-trimesityl-20-(4-thiocyanatophenyl)porphyrin is cleaved duringchromatography on alumina. Finally we subjected the S-acetyl-derivatizedporphyrin 20 to the same magnesium insertion conditions. Magnesiuminsertion occurred but with cleavage of the thiol protecting group,affording the free thiol Mg-21 in 32% yield (Scheme 5, FIG. 18).

The results of the gold-binding studies (vide infra) prompted us to usethe S-carbamoyl benzaldehyde 11 in subsequent syntheses. Thus mixedaldehyde-pyrrole condensations of aldehyde 11 with2,4,6-trimethoxybenzaldehyde, pentafluorobenzaldehyde, or n-hexanalyielded porphyrins 22, 23 or 24, respectively (Scheme 6, FIG. 19).Attempted conversion to the zinc chelate gave the thiol-protectedporphyrin Zn-22, however the more forcing conditions required formetalation of the tris(pentafluorophenyl)porphyrin andtri-n-pentyl-substituted porphyrin resulted in cleavage of theS-(N-ethylcarbamoyl) group, giving Zn-25 and Zn-26.

The design of porphyrins oriented in a horizontal manner can be achievedby the synthesis of porphyrins bearing a meta-(mercaptomethyl)phenylgroup at each of the four meso-positions. We attempted to repeat thesynthesis of the unprotected5,10,15,20-tetrakis[m-(mercaptomethyl)phenyl]porphyrin (Wen et al.(1997) J. Am. Chem. Soc., 119: 7726–7733), but encountered solubilityproblems due to disulfide formation. Because of the promising resultswith other thiol protecting groups cleaved directly on the gold surfacewe decided to synthesize the corresponding thiol-protected porphyrin. Asa protected sulfide entity we chose the thiocyanate group due to itshigh chemical stability.

Condensation of 10 with pyrrole at room temperature afforded the desired5,10,15,20-tetrakis[m-(thiocyanatomethyl)phenyl]porphyrin 27 as a darkpurple solid. Metalation with zinc acetate afforded the zinc-chelateZn-27 as a purple solid in 79% yield (Scheme 7, FIG. 20). Thethiocyanates were easily cleaved on the gold surface, affording aself-assembled porphyrin oriented parallel to the gold surface using allfour thiol groups for binding (vide infra).

Driven by these positive results we decided to synthesize a porphyrinwith only two ‘legs’ for attachment to the gold surface. Condensation ofaldehyde 10 with 5-phenyldipyrromethane (Lee and Lindsey (1994)Tetrahedron, 50: 11427–11440, Littler et al. (1999) J. Org. Chem., 64:1391–1396) using BF₃.OEt₂ in acetonitrile to minimize scrambling(Littler et al. (1999) J. Org. Chem., 64: 2864–2872) gave the desiredtrans-porphyrin 28 in 7% yield (accompanied by10,15,20-triphenyl-5-[m-(thiocyanatomethyl)phenyl]porphyrin in 2% yielddue to scrambling) (Scheme 8, FIG. 21). Metalation of 28 with zincacetate afforded the zinc porphyrin Zn-28 as a purple solid in 59%yield. Porphyrin Zn-28 also bound to the gold surface by in situcleavage of the thiocyanate units.

To achieve horizontally-oriented porphyrins with different potentials wedecided to metalate5,10,15,20-tetrakis[m-(S-acetylthiomethyl)phenyl]porphyrin (29) withvarious metal acetates (Scheme 9, FIG. 22). 29 was synthesized bysubstitution of all four bromides in5,10,15,20-tetrakis[m-(bromomethyl)phenyl)porphyrin (Wen et al. (1997)J. Am. Chem. Soc., 119: 7726–7733, Karaman et al. (1992) J. Am. Chem.Soc., 114: 4889–4898) with potassium thioacetate in 63% yield.Metalation of free base porphyrin 29 with zinc acetate gave the desiredzinc porphyrin Zn-29 in quantitative yield as a purple solid. UsingCo(OAc)₂.4H₂O afforded the corresponding cobalt chelate Co-29 as anorange-purple solid in quantitative yield.

Characterization.

The synthetic porphyrins are purple solids with a metallic reflection.The porphyrins are stable to air but slowly decompose in solution in thepresence of light. The purity of all porphyrin compounds was routinelychecked by TLC, ¹H NMR spectroscopy (with the exception of theparamagnetic Co-29), LD-MS and UV/VIS spectroscopy. Fluorescenceemission and excitation spectroscopy was used to confirm thecompleteness of the different metalation procedures. FAB-MS and IRspectra were measured to support the structure of the porphyrins.

Generally the LD-MS spectrum of a porphyrin shows the cationic moleculeion peak M⁺ in high intensity with only little fragmentation (Srinivasanet al. (1999) J. Porphyrins Phthalocyanines, 3: 283–291). But someporphyrins with delicate peripheral groups undergo characteristic andextensive fragmentation upon LD-MS analysis. Porphyrins with thiocyanatesubstituents show the loss of the cyano and the thiocyanato groups, withthe latter exhibiting more intense peaks. If more than one thiocyanategroup is present, fragmentation can occur for each of these groups.Porphyrins with S-acetyl groups show loss of both the acetyl (—COCH₃)and the thioacetate (—SCOCH₃) groups. Such fragmentation can generallyoccur for each thioacetate substituent. A further LD-MS feature observedwith thioacetate-derivatized porphyrins involves the appearance of an(M+15)⁺ peak. Because this peak occurred in the LD-MS spectra of allporphyrins with thioacetate substituents, which were synthesized viadifferent routes, and no other types of spectra show any evidence forthe presence of another species, this cannot be an impurity but must bea photochemical artifact involving the transfer of a methyl group. Ineach case the (M+15)⁺ peak exhibited the same pattern of fragmentationas observed for the parent molecule ion (M)⁺. The intensity of the(M+15)⁺ peak is about 10% of that of the M⁺ peak.

Behavior on Gold.

We surveyed the behavior of the thiol-protected zinc chelates Zn-15,Zn-16, Zn-17, Zn-18, Zn-19 and Zn-20 on gold electrodes. The members ofthis set of zinc porphyrins each bears three mesityl groups and differonly in the nature of the thiol protecting group. These studies revealedthat the S-(N-ethylcarbamoyl) (Zn-15) and S-acetyl (Zn-20) groups wereeasily cleaved on the gold surface, whereas the S-(2,4-dinitrophenyl)(Zn-16), S-(9-anthrylmethyl) (Zn-17), S-pivaloyl (Zn-18) and S-methyl(Zn-19) protecting groups were not cleaved. When no cleavage occurred,the thiol-protected porphyrins were not bound to the gold surface. Wefound that Zn-15 (S-(N-ethylcarbamoyl) protected), Zn-20 (S-acetylprotected), and Zn-21 (free thiol) bind to the gold surface identically.We also examined thiocyanatomethyl-derivatized porphyrins (Zn-27, Zn-28)on gold electrodes. We found that the thiocyanato protecting groupcleaves in situ and the corresponding thiol-derivatized porphyrin bindson the gold surface. These results are in accord with and extend aprevious report (Tour et al. (1995) J. Am. Chem. Soc. 117: 9529–9534)that the S-acetyl group of various thiol-substituted arenes (notporphyrins) is cleaved on the gold surface. Of the three thiolprotecting groups that we identified to undergo cleavage in situ on goldelectrodes, we also found in survey experiments that only the S-acetylgroup is compatible with Pd-coupling reactions for the preparation ofdiarylethyne-linked multiporphyrin arrays.

Porphyrins Zn-15, Zn-20, Zn-21, Zn-22, Zn-25, Zn-26, Zn-27, Zn-28, andMg-21 have been attached to a gold electrode. Porphyrins Zn-15, Zn-20,Zn-21, Zn-22, Zn-25, Zn-26, and Mg-21 bear one thiol or protected thioland bind to the gold surface in a vertical orientation. Porphyrins Zn-27and Zn-28 bind to the surface in a horizontal orientation with four andtwo sites of attachment, respectively. The set of zinc porphyrins withthree mesityl (Zn-15, Zn-20, Zn-21), n-pentyl (Zn-26),2,4,6-trimethoxyphenyl (Zn-22), or pentafluorophenyl (Zn-25) groups, aswell as the magnesium porphyrin with three mesityl groups (Mg-21),demonstrate the storage of data (upon oxidation) at differentelectrochemical potentials.

III. Experimental.

General.

All chemicals were obtained commercially and used as received unlessotherwise noted. Reagent grade solvents (CH₂Cl₂, CHCl₃, hexanes) andHPLC grade solvents (acetonitrile, toluene) were used as received fromFisher. Pyrrole was distilled from CaH₂. ¹H NMR spectra (300 MHz,General Electric GN 300NB), absorption spectra (HP 8453, Cary 3), andemission spectra (Spex FluoroMax) were collected routinely. All reportedNMR results were obtained at 300 MHz in CDCl₃. UV-Vis absorption spectrawere recorded in CH₂Cl₂ or toluene. Flash chromatography was performedon flash silica (Baker, 200–400 mesh) or alumina (Fisher, 80–200 mesh).Mass spectra were obtained via laser desorption (LD-MS) in the absenceof an added matrix (Fenyo et al. (1997) J. Porphyrins Phthalocyanines,1: 93–99) using a Bruker Proflex II mass spectrometer, fast atombombardment (FAB-MS) using a JEOL HX110HF mass spectrometer (ion source40° C., CsKI or polyethylene glycol standards, 10 ppm elementalcompositional accuracy for the porphyrins), or electron-impact massspectrometry (EI-MS).

2-[(4-Methylthio)phenyl]-5,5-dimethyl-1,3-dioxane (1).

Samples of 4-methylthiobenzaldehyde (20.0 mL, 150 mmol), neopentylglycol (16.0 g, 155 mmol), toluene (250 mL) and p-toluenesulfonic acid(190 mg, 1.00 mmol) were placed in a 500 mL flask fitted with aDean-Stark trap and a reflux condenser. The mixture was refluxedcautiously until a sudden exotherm ceased, then for an additional hour(total ˜1.5 h). The cooled mixture was washed with sodium bicarbonatesolution and with water. After drying with Na₂SO₄ and evaporation, whitecrystals crystallized from hexanes (32.2 g, 89.4%). mp 74–75° C.; ¹H NMR(CDCl₃) δ 0.83 (s, 3H, CH₃C), 1.33 (s, 3H, CH₃C), 2.50 (s, 3H, CH₃S),3.67 (AB/2, 2H, CH₂O, J=10.2 Hz), 3.79 (AB/2, 2H, CH₂O, J=10.2 Hz), 5.25(s, 1H, acetal), 7.30, (AA′BB′, 4H, ArH); ¹³C NMR (CDCl₃) δ 16.5, 22.6,23.8, 30.9, 78.3, 102.1, 127.1, 127.4, 136.2, 140.0; EI-MS m/z 238.1028(M)⁺ (C₁₃H₁₈O₂S requires 238.1028); Anal. Calcd. for C₁₃H₁₈O₂S: C,65.51; H, 7.61; S, 13.45; Found: C, 65.62; H, 7.70; S, 13.55.

2-[(4-Methylsulfoxy)phenyl]-5,5-dimethyl-1,3-dioxane (2).

A solution of acetal 1 (19 g, 80 mmol) in CH₂Cl₂ (150 mL) was cooled to−20° C. and stirred vigorously. Then a solution of MCPBA (31 g of 50–55%water suspension, 90 mmol) in CH₂Cl₂ (100 mL) was added dropwise over 1h. The mixture was stirred at 0° C. for an additional 1 h. Then Ca(OH)₂(11 g, 0.15 mmol) and Na₂SO₄ (20 g) were added and stirring wascontinued for 1 h. After filtration and evaporation, the warm colorlessoil was dissolved in CH₂Cl₂ (20 mL) and hexanes was added, affordingwhite crystals that were isolated by filtration (14.6 g). The filtratewas evaporated and the residual oil was recrystallized, affording asecond crop of white crystals. The total yield was 19.6 g (97%). mp116–117° C.; ¹H NMR (CDCl₃) δ 0.77 (s, 3H, CH₃C), 1.24 (s, 3H, CH₃C),2.65 (s, 3H, CH₃SO), 3.62 (AB/2, 2H, CH₂O, J=11.1 Hz), 3.74 (AB/2, 2H,CH₂O, J=10.8 Hz), 5.40 (s, 1H, acetal), 7.6–7.7 (m, 4H, ArH); ¹³C NMR(CDCl₃) δ 22.5, 23.7, 30.9, 44.7, 78.3, 101.3, 124.0, 128.0, 142.2,146.7; EI-MS obsd 254.0975, calcd exact mass 254.0977 (C₁₃H₁₈O₃S); Anal.Calcd. for C₁₃H₁₈O₃S: C, 61.39; H, 7.13; S, 12.61; Found: C, 61.29; H,7.03; S, 12.70.

2-(4-Mercaptophenyl)-5,5-dimethyl-1,3-dioxane (3).

Sulfoxide 2 (7.62 g, 30.0 mmol) was dissolved in CH₃CN (120 mL).2,6-Lutidine (10.8 mL, 93.0 mmol) was added and the mixture was cooledto −20° C. To the resulting suspension TFAA (12.7 mL, 90.0 mmol) wasadded dropwise maintaining the temperature below 0° C. The sulfoxidedisappeared and the mixture turned a lemon yellow. When the addition wascomplete, the mixture was stirred at ˜0° C. for 1 h. The mixture wasthen allowed to warm to room temperature. All volatile materials wereevaporated at 30° C. Next a precooled mixture of NEt₃ (50 mL) and MeOH(50 mL) was added. After 30 min at room temperature all volatilematerials were evaporated under reduced pressure at low temperature. Theresidual yellow oil was dissolved in ether (70 mL) and extracted withsat. NH₄Cl (250 mL). The layers were separated, the organic layer wasdried (Na₂SO₄) and concentrated to dryness giving a yellow-orange oil(6.61 g, 98% yield of crude material) of which ˜70% was the desiredcompound. The crude thiol was pure enough for the next step. A smallsample was oxidized to the respective disulfide and characterized. mp134–136° C.; ¹H NMR (CDCl₃) δ 0.80 (s, 3H, CH₃C), 1.28 (s, 3H, CH₃C),3.63 (AB/2, 2H, CH₂O, J=10.2 Hz), 3.76 (AB/2, 2H, CH₂O, J=11.2 Hz), 5.36(s, 1H, acetal), 7.43, 7.49 (AA′BB′, 4H, ArH); ¹³C NMR (CDCl₃) δ 16.5,22.6, 23.7, 30.9, 78.3, 101.8, 127.6, 128.1, 138.2, 138.4; FAB-MS obsd446.1574, calcd exact mass 446.1586 (C₂₄H₃₀O₄S₂); Anal. Calcd. forC₂₄H₃₀O₄S₂: C, 64.54; H, 6.77; S, 14.36; Found: C, 64.52; H, 6.70; S,14.44.

2-[(4-S-(N-Ethylcarbamoyl)thiophenyl]-5,5-dimethyl-1,3-dioxane (4).

To the crude thiol 3 (6.60 g, 29.5 mmol) was added ethyl isocyanoacetate(2.33 mL, 29.5 mmol) followed by phenylthiotrimethylsilane (0.568 mL,3.00 mmol). The reaction mixture was stirred for 3 h at roomtemperature. During this time the mixture gradually solidified to a paleyellow solid. Then n-pentane (5 mL) was added and the suspension wasfiltered and washed thoroughly with n-pentane. The yellowish crystalswere dissolved in hot toluene and hexanes was added. After standing fora few hours, off-white crystals were collected (4.01 g, yield 45.2% fromsulfoxide 2). mp 117–118° C.; ¹H NMR (CDCl₃) δ0.81 (s, 3H, CH₃C), 1.08(t, 3H, CH ³ —CH₂, J=7.2 Hz), 1.29 (s, 3H, CH₃C), 3.2–3.3 (m, 2H, CH₂N),3.67 (AB/2, 2H, CH₂O, J=10.8 Hz), 3.78 (AB/2, 2H, CH₂O, J=11.1 Hz), 5.42(s, 1H, acetal), 5.57 (bs, 1H, NH), 7.58 (bs, 4H, ArH); ¹³C NMR (CDCl₃)δ 15.5, 22.6, 23.7, 30.9, 37.2, 78.3, 101.5, 128.1, 130.0, 136.0, 140.7,166.4; EI-MS obsd 295.1235, calcd exact mass 295.1242 (C₁₅H₂₁NO₃S);Anal. Calcd. for C₁₅H₂₁NO₃S: C, 60.99; H, 7.17; N, 4.74; S, 10.86;Found: C, 61.16; H, 7.05; N, 4.70; S, 11.02.

2-[(4-S-(2,4-Dinitrophenyl)thiophenyl]-5,5-dimethyl-1,3-dioxane (5).

Crude thiol 3 (1.00 g, 4.46 mmol) was mixed with2,4-dinitrofluorobenzene (830 mg, 4.46 mmol). After heating to 35° C.,cesium fluoride (1.35 g, 8.92 mmol) was added. The yellow mixture wasstirred and heated at 45° C. for 1 h. Next toluene (10 mL) was added andthe hot suspension was filtered to remove insoluble materials. Thefiltrate was evaporated to dryness, giving an orange oil. The crudeproduct was chromatographed on silica (CH₂Cl₂/hexaness 1:2) affording ayellow oil, which finally was crystallized from hot ethanol, affordingyellow crystals (1.2 g, 63% from sulfoxide 2). mp 132–133° C.; ¹H NMR(CDCl₃) δ 0.77 (s, 3H, CH₃C), 1.24 (s, 3H, CH₃C), 3.63 (AB/2, 2H, CH₂O,J=10.8 Hz), 3.74 (AB/2, 2H, CH₂O, J=11.1 Hz), 5.40 (s, 1H, acetal), 6.93(d, 1H, ArH-5, J=8.7 Hz), 7.57 (AA′BB′, 4H, ArH), 8.00 (dd, 1H, ArH-6,J=8.7 Hz, J=2.1 Hz), 8.99 (d, 1H, ArH-3, J=2.1 Hz); ¹³C NMR (CDCl₃) δ22.4, 23.6, 30.9, 78.4, 101.2, 122.0, 127.5, 129.2, 129.6, 130.0, 136.4,142.2, 144.9, 148.8; EI-MS obsd 390.0873, calcd exact mass 390.0886(C₁₈H₁₈N₂O₆S); Anal. Calcd. for C₁₈H₁₈N₂O₆S: C, 55.38; H, 4.65; N, 7.18;S, 8.21; Found: C, 55.50; H, 4.64; N, 7.12; S, 8.30.

2-[(4-S-(9-Anthrylmethyl)thiophenyl]-5,5-dimethyl-1,3-dioxane (6).

Crude thiol 3 (1.15 g, 50.0 mmol) was dissolved in methanol (10 mL). Tothis solution was added a freshly prepared solution of sodium methoxide[from Na (117 mg, 50.0 mmol) and MeOH (50 mL)]. After 15 min the mixturewas evaporated to dryness and the orange solid was dried under vacuum.Then the solid was dissolved in anhydrous DMF (15 mL) at roomtemperature and a solution of 9-chloromethylanthracene (1.13 g, 50.0mmol) in DMF (10 mL) was added. The reaction mixture was stirred at roomtemperature for 72 h. The DMF was evaporated under reduced pressure, andthe resulting yellow oil was chromatographed on alumina(hexanes/CH₂Cl₂). The resulting yellow crystals were recrystallized fromCH₂Cl₂/hexanes to afford 1.17 g of the desired product (56.4%). mp158–159° C.; ¹H NMR (CDCl₃) δ 0.84 (s, 3H, CH₃C), 1.37 (s, 3H, CH₃C),3.69 (AB/2, 2H, CH₂O, J=10.8 Hz), 3.84 (AB/2, 2H, CH₂O, J=10.8 Hz), 5.01(s, 2H, CH₂S), 5.44 (s, 1H, acetal), 7.4–7.6 (m, 8H, anthracene), 8.01,8.26 (AA′BB′, 4H, ArH), 8.42 (s, 1H, anthracene); ¹³C NMR (CDCl₃) δ22.6, 23.8, 30.9, 32.7, 78.4, 102.0, 124.8, 125.8, 127.1, 127.6, 128.1,128.5, 129.7, 129.9, 130.8, 132.2, 137.5, 139.3; FAB-MS obsd 414.1653,calcd exact mass 414.1654 (C₂₇H₂₆O₂S); Anal. Calcd. for C₂₇H₂₆O₂S: C,78.22; H, 6.32; S, 7.73; Found: C, 78.05; H, 6.24; S, 7.63.

2-[(4-S-Pivaloylthiophenyl]-5,5-dimethyl-1,3-dioxane (7).

Crude thiol 3 (2.24 g, 10.0 mmol) was dissolved in CH₂Cl₂ (10 mL) andmethanol (10 mL) was added. To this solution was added a freshlyprepared solution of sodium methoxide [from Na (230 mg, 10.0 mmol) andMeOH (5 mL)]. After 30 min pivaloyl chloride (1.40 mL, 11.4 mmol) wasadded and the mixture was stirred for an additional 3 h at roomtemperature. After evaporation of all volatile components, the residualoil was chromatographed on silica, affording a mixture of less polarcompounds. The yellowish oil was further chromatographed usingcentrifugal preparative chromatography to afford a mixture of the titlecompound and the corresponding disulfide. The mixture was dissolved inCH₂Cl₂ and MeOH was added. Next CH₂Cl₂ was flushed out with argon. Thecrystals were filtered and dissolved in hot methanol and the mixture wascarefully cooled. After 30 min crystals of the title compound werecollected (502 mg, 16.0%). mp 115–116° C.; ¹H NMR (CDCl₃) δ 0.80 (s, 3H,CH₃C), 1.28 (s, 3H, CH₃C), 1.32 (s, 9H, C(CH₃)₃), 3.64 (AB/2, 2H, CH₂O,J=10.2 Hz), 3.77 (AB/2, 2H, CH₂O, J=10.2 Hz), 5.41 (s, 1H, acetal),7.40, 7.55 (AA′BB′, 4H, ArH); ¹³C NMR (CDCl₃) δ 22.6, 23.7, 28.1, 30.9,47.6, 78.3, 101.6, 127.5, 129.3, 135.4, 140.1; Anal. Calcd. forC₁₇H₂₄O₃S: C, 66.20; H, 7.84; S, 10.40; Found: C, 66.22; H, 7.92; S,10.60.

4-Thiocyanatobenzaldehyde (8).

Under an argon atmosphere, a mixture of 4-iodobenzaldehyde (232 mg, 1.00mmol), KSCN (95.0 mg, 1.00 mmol), CuSCN (120 mg, 1.00 mmol) and DMF (7.5mL) was heated with stirring in an oil bath maintained at 140° C. for 12h. After cooling, the mixture was diluted with toluene and water, andthen filtered through a Celite bed. The aqueous phase was extracted withtoluene, the organic fractions were combined and washed with water,dried and concentrated. The resulting dark oil was chromatographed onsilica gel using centrifugal preparative chromatography to obtainoff-white crystals (33 mg, 20%). mp 82–83° C.; ¹H NMR (CDCl₃) δ 7.63,7.92 (AA′BB′, 2H, ArH), 10.01 (s, 1H, CHO); ¹³C NMR (CDCl₃) δ 109.4,129.3, 131.6, 132.9, 191.3; EI-MS obsd 163.0092, calcd exact mass163.0092 (C₈H₅NOS); Anal. Calcd. for C₈H₅NOS: C, 58.88; H, 3.09; N,8.58; S, 19.65; Found: C, 58.85; H, 2.99; N, 8.61; S, 19.68.

4-S-Acetylthiobenzaldehyde (9).

4-Methylthiobenzaldehyde (4.45 mL, 0.033 mol) and sodium thiomethoxide(10 g, 0.13 mol) were suspended in HMPA (100 mL) and the reactionmixture was heating with stirring at 100° C. for 18 h. The resultingbrown suspension was cooled and acetyl chloride (10 mL) was added. After2 h the resulting suspension was poured into water and diethyl ether wasadded. The ethereal layer was extracted with water three times, driedand evaporated. Next chromatography was performed (silica,CH₂Cl₂/hexanes, 1:1). A yellow oil was collected containing the titlecompound with some impurities (3.33 g, crude yield 55.5%). This oil wasrecrystallized from ethanol giving off-white crystals (1.05 g, 18.3%).mp 44–45° C. (lit. 46° C., Zhdanov et al. (1970) Zh. Organ. Khim., 6:554–559, Engl. Transl. (1970), 6: 551–555); ¹H NMR (CDCl₃) δ 2.44 (s,3H, COCH3), 7.56 (AA′BB′, 2H, ArH), 7.87 (AA′BB′, 2H, ArH), 10.00 (s,1H, CHO); ¹³C NMR (CDCl₃) δ 31.2, 130.6, 135.2, 136.1, 137.1, 192.1,192.9; Anal. Calcd. for C₉H₈O₂S: C, 59.98; H, 4.47; S, 17.79; Found: C,59.58; H, 4.52; S, 17.78.

m-(Thiocyanatomethyl)benzaldehyde (10).

To a solution of 300 mg of m-(bromomethyl)benzaldehyde (Wagner et al.(1997) Tetrahedron, 53: 6755–6790) (1.5 mmol) in 5 mL of methanol wasadded a solution of 321 mg of potassium thiocyanate (3.3 mmol) in 4 mLof methanol under stirring at ambient temperature. After a few minutes aprecipitate formed. The reaction was monitored by TLC and stopped byadding 20 mL of H₂O when no starting material was detectable. 30 mL ofether was added and the phases were separated. The aqueous phase waswashed twice with 20 mL of ether and the combined organic phases weredried (Na₂SO₄). Column chromatography over flash silica gel withether/hexanes (1:2) gave 198 mg (1.1 mmol, 74% yield) of a slightlyyellow oil which solidified upon standing at 0° C. Recrystallization(ether/hexanes) gave colorless crystals (mp 39° C.). IR (neat): {tildeover (v)}=3060 cm⁻¹ (m, arom. CH), 2996 (m, CH), 2832 (s, CH), 2751 (m,CH), 2149 (s, CN), 1695 (s, C═O), 1603 (s, arom. C═C), 1450 (m, arom.C═C), 1424 (m), 1394 (w), 1294 (w), 1145 (s), 1084 (w), 1006 (w), 960(w), 908 (w), 881 (w), 803 (s), 754 (w), 696 (s), 652 (s); ¹H NMR (300.5MHz, CDCl₃): δ=4.22 (s, 2H, CH₂), 7.55–7.69 (m, 2H, ArH), 7.86–7.93 (m,ArH), 10.04 (s, 1H, CHO); ¹³C NMR (75.6 MHz, CDCl₃, APT): δ=32.7 (+,CH₂), 111.4 (+, CN), 129.6 (−, CH), 129.8 (−, CH), 130.1 (−, CH), 134.6(−, CH), 135.7 (+, C_(q)), 136.8 (+, C_(q)), 191.4 (−, CHO); GC-MS (EI)obsd 177 [M⁺], 149 [M⁺−CO], 120, 119 [M⁺−SCN], 91 [M⁺−SCN—CO], 90, 89,77 [C₆H₅ ⁺], 65, 63; Anal. Calcd for C₉H₇NOS, C, 60.99; H, 3.98; N,7.90; S, 18.09. Found: C, 60.75; H, 4.05; N, 7.86; S, 18.19.

General Procedure for Synthesis of Porphyrins 15–18, 19–20 and 22–24.

Acetal (4, 5, 6 or 7) (0.730 mmol) was dissolved in CH₂Cl₂ (1 mL) andtrifluoroacetic acid (2 mL) was added. The mixture was stirred at roomtemperature overnight. After evaporation of the reaction mixture todryness, the residue was redissolved in CHCl₃ (40 mL). Alternatively,aldehyde 9 or 4-methylthiobenzaldehyde (0.730 mmol) was added to CHCl₃(40 mL). Next samples of the other aldehyde (2.20 mmol), pyrrole (0.200mL, 2.92 mmol) and BF₃.OEt₂ (0.090 mL, 0.71 mmol) were added. Thereaction mixture was stirred at room temperature for 90 min. Then DDQ(500 mg, 2.20 mmol) was added and the reaction mixture was gentlyrefluxed for 1 h. After cooling, the reaction mixture was passed over ashort silica column (CH₂Cl₂) affording porphyrins usually free from darkpigments and quinone species. Further purification details are describedfor each case as follows.

5,10,15-Trimesityl-20-[4-S-(N-ethylcarbamoyl)thiophenyl]porphyrin (15).

The mixture of porphyrins was loaded onto a silica column (4×30 cm,toluene). The title porphyrin comprised the second purple band,affording 72 mg (12%). ¹H NMR (CDCl₃) δ −2.47 (s, 2H, NHpyrrole), 1.32(t, 3H, CH ³ —CH₂, J=7.2 Hz), 1.94 (s, 18H, ArCH₃), 2.69 (s, 9H, ArCH₃),3.4–3.6 (m, 2H, CH₂N), 5.65 (bs, 1H, NH), 7.35 (s, 6H, ArH), 8.00, 8.33(AA′BB′, 4H, ArH), 8.72 (s, 4H, β-pyrrole), 8.78 (d, 2H, β-pyrrole,J=4.2 Hz), 8.87 (d, 2H, β-pyrrole, J=4.2 Hz); LD-MS calcd av mass 844.1,obsd 844.6 [M⁺], 773.1 [M⁺−CONHEt]; FAB-MS obsd 843.4019, calcd exactmass 843.3971 (C₅₆H₅₃N₅OS); λ_(abs) (CH₂Cl₂) 419, 515, 548, 591 nm.

5,10,15-Trimesityl-20-[4-S-(2,4-dinitrophenyl)thiophenyl]porphyrin (16).

The mixture of porphyrins was purified by preparative centrifugal TLC(silica, toluene/hexanes, 1:2). The title porphyrin comprised the secondpurple band, affording 70 mg (10%). ¹H NMR (CDCl₃) δ −2.44 (s, 2H,NHpyrrole), 1.95 (s, 18H, ArCH₃), 2.71 (s, 9H, ArCH₃), 7.30 (d, 1H, ArH,J=8.1 Hz), 7.37 (s, 6H, ArH), 7.60 (d, 1H, ArH, J=8.7 Hz), 8.00, 8.49(AA′BB′, 4H, ArH), 8.75 (s, 4H, β-pyrrole), 8.8–9.0 (m, 4H, β-pyrrole),9.30 (d, 1H, ArH, J=2.1 Hz); LD-MS calcd av mass 938.4, obsd 938.0;FAB-MS obsd 938.3632, calcd exact mass 938.3614 (C₅₉H₅₀N₆O₄S); λ_(abs)(CH₂Cl₂) 419, 515, 549, 591, 646 nm.

5,10,15-Trimesityl-20-[4-S-(9-anthrylmethyl)thiophenyl]porphyrin (17).

The mixture was chromatographed on an alumina column (toluene/hexanes,1:4). The resulting mixture of porphyrins was purified by preparativecentrifugal TLC (silica, toluene/hexanes, 1:3). The title porphyrincomprised the second purple band, affording 28 mg (4.0%). ¹H NMR (CDCl₃)δ −2.51 (s, 2H, NHpyrrole), 1.90 (s, 18H, ArCH₃), 2.66 (s, 9H, ArCH₃),5.43(s, 2H, CH₂S), 7.1–7.8 (m, 5H, anthracene), 7.31 (s, 6H, ArH), 7.79,8.52 (AA′BB′, 4H, ArH), 8.0–8.2 (m, 4H, anthracene), 8.68 (s, 4H,β-pyrrole), 8.75 (d, 2H, β-pyrrole, J=4.5 Hz), 8.80 (d, 2H, β-pyrrole,J=4.5 Hz); LD-MS calcd av mass 962.5, obsd 964.0, 787.4 [M⁺−C₁₄H₈],773.1 [M⁺−C₁₅H₁₀]; FAB-MS obsd 962.4368, calcd exact mass 962.4382(C₆₈H₅₈N₄S); λ_(abs) (CH₂Cl₂) 420, 515, 549, 593, 648 nm.

5,10,15-Trimesityl-20-[4-S-pivaloyl-thiophenyl]porphyrin (18).

The mixture was chromatographed on a silica column (toluene/hexanes,1:1).

The title porphyrin comprised the second purple band, affording 68 mg(11%). ¹H NMR (CDCl₃) δ −2.49 (s, 2H, NH), 1.52 (s, 9H, C(CH₃)₃), 1.92(s, 18H, ArCH₃), 2.68 (s, 9H, ArCH₃), 7.35 (s, 6H, ArH), 7.83, 8.30(AA′BB′, 4H, ArH), 8.70 (s, 4H, β-pyrrole), 8.75 (d, 2H, β-pyrrole,J=5.4 Hz), 8.88 (d, 2H, β-pyrrole, J=5.4 Hz); LD-MS calcd av mass 856.4,obsd 858.6, 831.5 [M⁺−C₂H₆], 774.2 [M⁺−COC(CH₃)₃]; FAB-MS obsd 856.4186,calcd exact mass 856.4175 (C₅₈H₅₆N₄OS); λ_(abs) (CH₂Cl₂) 419, 515, 548,591, 646 nm.

5,10,15-Trimesityl-20-[4-S-methylthiophenyl]porphyrin (19).

The mixture was chromatographed on a silica column (toluene/hexanes,1:1). The resulting mixture was next chromatographed on silica column(toluene/hexanes, 1:4). The title porphyrin comprised the second purpleband, affording 57 mg (10%). ¹H NMR (CDCl₃) δ −2.49 (s, 2H, NH), 1.92(s, 18H, ArCH₃), 2.68 (s, 9H, ArCH₃), 2.79 (s, 3H, SCH₃), 7.33 (s, 6H,ArH), 7.67, 8.18 (AA′BB′, 4H, ArH), 8.70 (s, 4H, β-pyrrole), 8.74 (d,2H, β-pyrrole, J=5.1 Hz), 8.87 (d, 2H, β-pyrrole, J=5.1 Hz); LD-MS calcdav mass 786.4, obsd 786.9; FAB-MS obsd 786.3790, calcd exact mass786.3756 (C₅₄H₅₀N₄S); λ_(abs) (CH₂Cl₂) 420, 515, 550, 592, 648 nm.

5,10,15-Trimesityl-20-[4-S-acetylthiophenyl]porphyrin (20).

The mixture was chromatographed on a silica column (toluene/hexanes 1:1,then toluene). The title porphyrin comprised the second purple band,affording 62 mg (10.5%). ¹H NMR (CDCl₃) δ −2.46 (s, 2H, NH), 1.94 (s,18H, ArCH₃), 2.66 (s, 3H, COCH₃), 2.70 (s, 9H, ArCH₃), 7.35 (s, 6H, ArH,7.88, 8.35 (AA′BB′, 4H, ArH), 8.73 (s, 4H, β-pyrrole), 8.79 (d, 2H,β-pyrrole, J=4.2 Hz), 8.89 (d, 2H, β-pyrrole, J=4.2 Hz); LD-MS calcd avmass 814.4, obsd 815.7, 813.9 [M⁺+15]; 787.7 [M⁺−CH₃CO+15], 773.7[M⁺−CH₃CO]; FAB-MS obsd 814.3694, calcd exact mass 814.3705(C₅₅H₅₀N₄OS); λ_(abs) (CH₂Cl₂) 419, 515, 548, 591, 647 nm.

5,10,15-Tris(2,4,6-trimethoxyphenyl)-20-[4-S-(N-ethylcarbamoyl)thiophenyl]porphyrin(22).

Purification was performed by preparative centrifugal chromatography(silica, CH₂Cl₂/MeOH, 98:2). The title compound was obtained as a ˜1:1mixture with 5,10,15,20-tetrakis(2,4,6-trimethoxyphenyl)porphyrin. Thepresence of the title compound was confirmed by mass spectrometry (LD-MSC₅₆H₅₃N₅O₁₀S calcd av mass 987.4, obsd 988.6). This mixture was notpurified further but was used in the metalation reaction to prepareZn-22.

5,10,15-Tris(2,3,4,5,6-pentafluorophenyl)-20-[4-S-(N-ethylcarbamoyl)thiophenyl]-porphyrin(23).

The mixture of porphyrins was chromatographed on a silica column (4×30cm, hexanes/CH₂Cl₂, 2:1). The title porphyrin comprised the secondpurple band, affording 72 mg (10%). ¹H NMR (CDCl₃) δ −2.74 (s, 2H,NHpyrrole), 1.32 (t, 3H, CH ³ —CH₂, J=7.2 Hz), 3.5–3.6 (m, 2H, CH₂N),5.67 (bs, 1H, NH), 8.00, 8.32 (AA′BB′, 4H, ArH), 8.94 (d, 2H, β-pyrrole,J=5.1 Hz), 9.02 (s, 4H, β-pyrrole), 9.09 (d, 2H, β-pyrrole, J=4.2 Hz);LD-MS calcd av mass 987.1, obsd 989.9, 918.7 [M⁺−CONHEt]; FAB-MS obsd987.1136, calcd exact mass 987.1149 (C₄₇H₂₀F₁₅N₅OS); λ_(abs) (CH₂Cl₂)415, 509, 540, 584, 638 nm.

5,10,15-Tri-n-pentyl-20-[4-S-(N-ethylcarbamoyl)thiophenyl]porphyrin(24).

The free base was purified by preparative centrifugal chromatography(silica/CH₂Cl₂/hexanes, 5:1) followed by column chromatography(silica/CH₂Cl₂/toluene, 4:1). The title porphyrin comprised the secondpurple band, affording 9 mg (4%). ¹H NMR (CDCl₃) δ −2.62 (s, 2H, NH),1.00–1110 (m, 9H, CH ³ —CH₂), 1.30–1.70 (m, 9H, CH₂ aliphatic+CH ³—CH₂—N), 1.75–1.90 (m, 6H, CH₂), 2.45–2.70 (m, 6H, CH₂), 3.50–3.62 (m,4H, N—CH₂), 4.90–5.10 (m, 6H, CH₂-porphyrin), 5.63 (bt, 1H, NH, J=5.1Hz), 7.98, 8.26 (AA′BB′, 4H, ArH), 8.85 (d, 2H, β-pyrrole, J=4.2 Hz),9.43 (d, 2H, β-pyrrole, J=5.1 Hz); 9.52–9.62 (m, 4H, β-pyrrole); LD-MScalcd av mass 699.4, obsd 700.7; FAB-MS obsd 699.3996, calcd exact mass699.3971 (C₄₄H₅₃N₅OS); λ_(abs) (CH₂Cl₂) 419, 519, 554, 598, 656 nm.

General Procedure for Zinc Insertion.

Porphyrin (0.040 mmol) was dissolved in CH₂Cl₂ (15 mL) and a solution ofZn(OAc)₂.2H₂O (880 mg, 4.00 mmol) in methanol (15 mL) was added. Thereaction mixture was stirred overnight at room temperature. Aftermetalation was complete (TLC, fluorescence excitation spectroscopy), thereaction mixture was washed with water and 10% NaHCO₃, dried (Na₂SO₄),filtered and rotary evaporated to a purple solid. Purification wasachieved by chromatography on silica.

Zn(II)-5,10,15-Trimesityl-20-[4-S-(N-ethylcarbamoyl)thiophenyl]porphyrin(Zn-15).

Column chromatography (silica, CH₂Cl₂) afforded 29 mg (75%). ¹H NMR(CDCl₃) δ 1.30 (t, 3H, CH ³ —CH₂, J=7.5 Hz), 1.87 (s, 18H, ArCH₃), 2.66(s, 9H, ArCH₃), 3.4–3.6 (m, 2H, CH₂N), 5.61 (bs, 1H, NH), 7.31 (s, 6H,ArH), 7.93, 8.30 (AA′BB′, 4H, ArH), 8.74 (s, 4H, β-pyrrole), 8.80 (d,2H, β-pyrrole, J=5.1 Hz), 8.88 (d, 2H, β-pyrrole, J=4.2 Hz); LD-MS calcdav mass 905.3, obsd 906.7, 835.7 [M⁺—CONHEt]; FAB-MS obsd 905.3098,calcd exact mass 905.3106 (C₅₆H₅₁N₅OSZn); λ_(abs) (CH₂Cl₂) 421, 549 nm.

Zn(II)-5,10,15-Trimesityl-20-[4-S-(2,4-dinitrophenyl)thiophenyl]porphyrin(Zn-16).

Column chromatography (silica, toluene/hexanes) afforded 34 mg (85%). ¹HNMR (CDCl₃) δ 1.87 (s, 18H, ArCH₃), 2.64 (s, 9H, ArCH₃), 7.26 (d, 1H,ArH, J=9.0 Hz), 7.29 (s, 6H, ArH), 7.54 (d, 1H, ArH, J=9.0 Hz), 7.99,8.44 (AA′BB′, 4H, ArH), 8.75 (s, 4H, β-pyrrole), 8.86 (AB, 4H,β-pyrrole, J=4.5 Hz), 9.23 (d, 1H, ArH, J=3.0 Hz); LD-MS calcd av mass1000.3, obsd 1000.3; FAB-MS obsd 1000.2726, calcd exact mass 1000.2749(C₅₉H₄₈N₆O₄SZn); λ_(abs) (CH₂Cl₂) 422, 550 nm.

Zn(II)-5,10,15-Trimesityl-20-[4-S-(9-anthrylmethyl)thiophenyl]porphyrin(Zn-17).

The product was purified by preparative centrifugal TLC (silica,hexanes/CH₂Cl₂) 31 mg (74%). ¹H NMR (CDCl₃) δ 1.85 (s, 9H, ArCH₃), 1.88(s, 9H, ArCH₃), 2.65 (s, 9H, ArCH₃), 5.36 (s, 2H, CH₂S), 7.1–8.5 (m,19H, anthracene+ArH), 8.72 (s, 2H, β-pyrrole), 8.73 (s, 2H, β-pyrrole),8.8–9.0 (m, 4H, β-pyrrole); LD-MS calcd av mass 1024.4, obsd 1027.3,834.9 [M⁺−C₁₅H₁₀]; FAB-MS obsd 1024.3529, calcd exact mass 1024.3517(C₆₈H₅₆N₄SZn); λ_(abs) (CH₂Cl₂) 421, 550 nm.

Zn(II)-5,10,15-Trimesityl-20-[4-S-pivaloylthiophenyl]porphyrin (Zn-18).

The product was purified on a silica column (toluene/hexanes, 1:1),affording 31 mg (85%). ¹H NMR (CDCl₃) δ 1.50 (s, 9H, C(CH₃)₃), 1.87 (s,18H, ArCH₃), 2.65 (s, 9H, ArCH₃), 7.31 (s, 6H, ArH), 7.79, 8.29 (AA′BB′,4H, ArH), 8.74 (s, 4H, β-pyrrole), 8.79 (d, 2H, β-pyrrole, J=4.2 Hz),8.91 (d, 2H, β-pyrrole, J=4.2 Hz); LD-MS calcd av mass 918.3, obsd919.5, 891.4 [M⁺−C₂H₆], 835.3 [M⁺−COC(CH₃)₃]; FAB-MS obsd 918.3332,calcd exact mass 918.3310 (C₅₈H₅₄N₄OSZn); λ_(abs) (CH₂Cl₂) 422, 549 nm.

Zn(II)-5,10,15-Trimesityl-20-[4-S-methylthiophenyl]porphyrin (Zn-19).

The mixture was chromatographed on a silica column (toluene/hexanes,1:1). The resulting mixture was next chromatographed on a silica column(toluene/hexanes, 1:4). The title porphyrin comprised the second purpleband, affording 31 mg (90%). ¹H NMR (CDCl₃) δ 1.84 (s, 18H, ArCH₃), 2.63(s, 9H, ArCH₃), 2.74 (s, 3H, SCH₃), 7.26 (s, 6H, ArH), 7.59, 8.13(AA′BB′, 2H1, ArH), 8.70 (s, 4H, β-pyrrole), 8.75 (d, 2H, β-pyrrole,J=5.1 Hz), 8.87 (d, 2H, β-pyrrole, J=5.1 Hz); LD-MS calcd av mass 848.3,obsd 851.5; FAB-MS obsd 848.2913, calcd exact mass 848.2891(C₅₄H₄₈N₄SZn); λ_(abs) (CH₂Cl₂) 421, 550 nm.

Zn(II)-5,10,15-Trimesityl-20-[4-S-acetylthiophenyl]porphyrin (Zn-20).

Method 1.

(From 20 by general zinc insertion procedure). Purification bychromatography (silica, toluene/CH₂Cl₂). Yield 82%.

Method 2.

Zn(II)-porphyrin Zn-15 (9.0 mg, 0.010 mmol) was dissolved in CH₂Cl₂ (20mL) and the solution was carefully flushed with argon. Next a solutionof sodium methoxide [freshly prepared from sodium (23 mg, 1.0 mmol) andMeOH (10 mL) under argon] was added. The reaction mixture was stirredunder argon at room temperature for 1 h. Next acetyl chloride (1 mmol,0.7 mL) was added and the mixture was evaporated to dryness. The mixtureof porphyrins was dissolved in CH₂Cl₂ and chromatographed (silica,hexanes/CH₂Cl₂) affording 6.3 mg (72%). ¹H NMR (CDCl₃) δ 1.85 (s, 18H,ArCH₃), 2.63 (s, 9H, ArCH₃), 2.61 (s, 3H, CH₃CO), 7.28 (s, 6H, ArH),7.46, 8.07 (AA′BB′, 4H, ArH), 8.70 (s, 4H, β-pyrrole), 8.74 (d, 2H,β-pyrrole, J=5.1 Hz), 8.83 (d, 2H, β-pyrrole, J=5.1 Hz); LD-MS calcd avmass 876.3, obsd 874.6; FAB-MS obsd 878.2983, calcd exact mass 878.2997(C₅₅H₅₀N₄OSZn).

Zn(II)-5,10,15-Trimesityl-20-[4-mercaptophenyl]porphyrin (Zn-21).

A sample of Zn-15 (9.0 mg, 0.010 mmol) was dissolved in CH₂Cl₂ (20 mL)and the solution was carefully flushed with argon. Next a solution ofsodium methoxide [freshly prepared from sodium (23 mg, 1 mmol) and MeOH(10 mL) and also flushed with argon] was added. The reaction mixture wasstirred at room temperature for 1 h. Next HCl (0.2 mL, 5 M soln.) wasadded and the mixture was evaporated to dryness. The mixture ofporphyrins was dissolved in CH₂Cl₂ and chromatographed (silica,hexanes/CH₂Cl₂) affording 4.3 mg (51%). ¹H NMR (CDCl₃) δ 1.89 (s, 18H,ArCH₃), 2.66 (s, 9H, ArCH₃), 7.31 (s, 6H, ArH), 7.64, 8.22 (AA′BB′, 2H,ArH), 8.76 (s, 4H, β-pyrrole), 8.8–9.0 (m, 4H, β-pyrrole); LD-MS calcdav mass 834.3, obsd 834.0; FAB-MS obsd 834.2071, calcd exact mass834.2735 (C₅₃H₄₆N₄SZn).

Zn(II)-5,10,15-Tris(2,4,6-trimethoxyphenyl)-20-[4-S—(N-ethylcarbamoyl)thiophenyl]-porphyrin(Zn-22).

30.0 mg of a mixture of the desired free base A₃B-porphyrin and thecorresponding A₄-porphyrin was metalated according to the generalprocedure. The desired chelate was purified by preparative centrifugalchromatography (silica/CH₂Cl₂/MeOH, 99:1), affording 10.0 mg (1.3% fromacetal 5). ¹H NMR (CDCl₃) δ 1.27 (t, 3H, CH ³ —CH₂, J=7.2 Hz), 3.4–3.6(m, 2H, CH₂N), 3.49 (s, 18H, OCH₃), 4.09 (s, 9H, OCH₃), 5.57 (bt, 1H,NH, J=5.1 Hz), 6.57 (s, 6H, ArH), 7.89, 8.26 (AA′BB′, 4H, ArH), 8.81 (d,2H, β-pyrrole, J=4.5 Hz), 8.86 (d, 2H, β-pyrrole, J=4.5 Hz), 8.84 (s,4H, β-pyrrole); LD-MS calcd av mass 1049.3, obsd 1052.7, 981.5[M⁺−CONHEt]; FAB-MS obsd 1049.2666, calcd exact mass 1049.2648(C₅₆H₅₁N₅O₁₀SZn); λ_(abs) (CH₂Cl₂) 423, 549 nm

Zn(II)-5,10,15-Tris(2,3,4,5,6-pentafluorophenyl)-20-[4-mercaptophenyl]porphyrin(Zn-25).

Refluxing a mixture of porphyrin 25 and Zn(OAc)₂.2H₂O for 8 h followedby purification on silica (CH₂Cl₂) afforded 25 mg (63% yield). ¹H NMR(CDCl₃) δ 3.79 (s, 1H, SH), 7.67, 8.09 (AA′BB′, 4H, ArH), 8.93 (d, 2H,β-pyrrole, J=4.2 Hz), 9.00 (s, 4H, β-pyrrole), 9.09 (d, 2H, β-pyrrole,J=4.2 Hz); LD-MS calcd av mass 978.0, obsd 975.4; FAB-MS obsd 977.9925,calcd exact mass 977.9913 (C₄₄H₁₃F₁₅N₄SZn). λ_(abs) (CH₂Cl₂) 416, 545nm.

Zn(II)-5,10,15-Tri-n-pentyl-20-[4-mercaptophenyl]porphyrin (Zn-26).

The product was purified by column chromatography (silica/CH₂Cl₂)followed by preparative centrifugal TLC (silica, hexanes/CH₂Cl₂, 1:9),affording 12 mg (44%). ¹H NMR (CDCl₃) δ 0.80–1.40 (m, 12H, CH ³ —CH₂+CH³ —CH₂—N), 1.50–1.70 (m, 6H, CH₂ aliphatic), 1.75–1.95 (m, 6H, CH₂),2.40–2.60 (m, 6H, CH₂), 4.70–4.90 (m, 6H, CH₂-porphyrin), 8.17, 8.28(AB′BB′, 4H, ArH), 8.95 (d, 2H, β-pyrrole, J=5.1 Hz), 9.35–9.48 (d, 6H,β-pyrrole); LD-MS calcd av mass 690.3, obsd 690.4, 633.1 [M⁺−C₄H₉];FAB-MS obsd 690.2706, calcd exact mass 690.2735 (C₄₁H₄₆N₄SZn); λ_(abs)(CH₂Cl₂) 419, 554 nm.

Mg(II)-5,10,15-Trimesityl-20-[4-mercaptophenyl]porphyrin (Mg-21).

Porphyrin 20 (16 mg, 0.020 mmol) was dissolved in CH₂Cl₂ (5 mL) and MgI₂(56 mg, 0.20 mmol) and DIEA (0.070 mL, 0.40 mmol) were added. After 10min the mixture was diluted with CH₂Cl₂ (20 mL), washed with 10% NaHCO₃and dried. The resulting pink-violet residue was chromatographed on analumina column (CH₂Cl₂/MeOH, 100:1, 100:2, 100:4) to afford thepink-violet product (5.0 mg, 31%). ¹H NMR (CDCl₃) δ 1.80 (s, 18H,ArCH₃), 2.61 (s, 9H, ArCH₃), 7.23 (s, 6H, ArH), 7.8–8.1 (m, 4H, ArH),8.5–8.9 (m, 8H, β-pyrrole); LD-MS calcd av mass 795.3, obsd 797.4;FAB-MS obsd 794.3278, calcd exact mass 794.3294 (C₅₃H₄₆N₄SMg); λ_(abs)(CH₂Cl₂) 426, 565, 605 nm.

5,10,15,20-Tetrakis[m-(thiocyanatomethyl)phenyl]porphyrin (27).

A solution of 513 mg of m-(thiocyanatomethyl)benzaldehyde (10, 2.9 mmol)and 0.20 mL of pyrrole (193 mg, 2.9 mmol) in 300 mL of CHCl₃ was purgedwith argon for 30 min. Under stirring at ambient temperature 12 μL ofBF₃.O(Et)₂ (13 mg, 0.1 mmol) and 180 μL of TFA (266 mg, 2.3 mmol) wereadded. Soon the solution turned yellow and later to dark red. After 2 han additional 90 μL of BF₃.O(Et)₂ (98 mg, 0.7 mmol) was added. After 2 h500 μL of TEA (364 mg, 3.6 mmol) and 583 mg of o-tetrachlorobenzoquinone(2.4 mmol) were added and the mixture was refluxed for 1 h. The mixturewas cooled to room temperature and the solvents were removed underreduced pressure. Column chromatography over flash silica gel withether/hexanes (3:1) gave 119 mg (0.1 mmol, 18% yield) of a dark purplesolid. IR (neat) {tilde over (v)}=2953 (m, CH), 2917 (s, CH), 2846 (m,CH), 2152 (m, CN), 1470 (m), 1392 (w), 1344 (w), 1152 (w), 1082 (w); ¹HNMR (300.5 MHz, CDCl₃): δ=−2.84 (s, 2H, NH), 4.44 (s, 8H, CH₂),7.76–7.85 (m, 8H, ArH), 8.18–8.25 (m, 8H, ArH), 8.88 (s, 8H, β-pyrrole);LD-MS calcd av mass 899.1, obsd 901.7, 876.3 [M⁺−CN], 843.5 [M⁺−SCN],818.1 [M⁺−SCN—CN], 785.1 [M⁺−2 SCN], 758.5 [M⁺−2 SCN—CN], 726.8 [M⁺−3SCN], 668.0 [M⁺−4 SCN], 577.6 [M⁺−4 SCN—C₇H₇]; FAB-MS obsd 898.1797,calcd exact mass 898.1789 (C₅₂H₃₄N₈S₄); λ_(abs) (toluene) 420, 514, 549,590, 646 nm.

Zinc(II)-5,10,15,20-Tetrakis[m-(thiocyanatomethyl)phenyl]porphyrin(Zn-27).

To a solution of 84 mg of5,10,15,20-tetrakis[m-(thiocyanatomethyl)phenyl]porphyrin (27, 93 μmol)in 50 mL of CHCl₃ was added 250 mg of Zn(OAc)₂.2H₂O (1.1 mmol) in 5 mLof methanol under stirring at ambient temperature. After completion ofthe metalation (checked by fluorescence excitation spectroscopy) themixture was washed with 20 mL of 10% NaHCO₃ and 20 mL of H₂O, dried(Na₂SO₄) and filtered. The solvents were removed under reduced pressureaffording 71 mg (74 μmol, 79% yield) of a dark purple solid.Recrystallization (CH₂Cl₂/methanol) gave dark purple crystals. IR(neat): {tilde over (v)}=2995 (m, CH), 2880 (w, CH), 2153 (m, CN), 1652(w), 1601 (m, arom. C═C), 1478 (m), 1436 (m), 1338 (m), 1206 (m), 1070(w), 1031 (w), 1001 (s), 934 (m), 795 (s), 707 (s); ¹H NMR (300.5 MHz,CDCl₃): δ=4.44 (s, 8H, CH₂), 7.77–7.83 (m, 8H, ArH), 8.20–8.25 (m, 8H,ArH), 8.97 (s, 8H, β-pyrrole); LD-MS calcd av mass 962.5, obsd 958.2,900.2 [M⁺−SCN], 843.5 [M⁺−2 SCN], 787.5 [M⁺−3 SCN], 726.7 [M⁺−4 SCN];FAB-MS obsd 960.0959, calcd exact mass 960.0924 (C₅₂H₃₂N₈S₄Zn); λ_(abs)(toluene) 422, 550, 589 nm; λ_(em) (toluene) 603, 652 nm.

10,20-Diphenyl-5,15-bis[-m-(thiocyanatomethyl)phenyl]porphyrin (28).

A mixture of 316 mg of m-(thiocyanatomethyl)benzaldehyde (10, 1.8 mmol),396 mg of 5-phenyldipyrromethane (Lee and Lindsey (1994) Tetrahedron,50: 11427–11440, Littler et al. (1999) J. Org. Chem., 64: 1391–1396)(1.8 mmol) and 1.07 g of NH₄Cl (20.0 mmol) in 200 mL of acetonitrile waspurged with argon for 30 min. Under stirring at ambient temperature 23μL of BF₃.O(Et)₂ (26 mg, 0.18 mmol) was added. Soon the solution turnedto yellow and later to dark red. After 6.5 h, 607 mg of DDQ (2.7 mmol)was added. After 1 h the reaction was quenched with 0.5 mL of TEA (365g, 3.6 mmol). The solvents were removed under reduced pressure.Purification was done by column chromatography over two flash silica gelcolumns with different solvent mixtures: (column 1) ether/hexanes (3:1)and (column 2) CH₂Cl₂/hexanes (gradient, start: 1:1). Two fractions ofdark purple solids were obtained. I: 12 mg10,15,20-triphenyl-5-[m-(thiocyanatomethyl)phenyl]porphyrin (17.5 μmol,2% yield). II: 44 mg of the title compound (58.1 μmol, 7% yield). IR(neat): {tilde over (v)}=2921 (s, CH), 2850 (m, CH), 2154 (m, CN), 1597(m, arom. C═C), 1471 (s), 1348 (m), 1206 (m), 1181 (w), 1097 (w), 973(s), 898 (w), 743 (s), 691 (s), 623 (s); ¹H NMR (300.5 MHz, CDCl₃):δ=−2.811 (s, 2H, NH), 4.57 (s, 4H, CH₂), 7.71–7.83 (m, 11H, ArH),8.17–8.25 (m, 8H, ArH), 8.84 (d, 4H, β-pyrrole, ³J=5.1 Hz), 8.88 (d, 4H,β-pyrrole); LD-MS calcd av mass 756.9, obsd 757.4, 699.2 [M⁺−SCN], 641.0[M⁺−2 SCN]; FAB-MS obsd 756.2172, calcd exact mass 756.2130(C₄₈H₃₂N₆S₂); λ_(abs) (toluene) 420, 514, 549, 590, 647, 657 nm.

Zinc(II)-10,20-Diphenyl-5,15-bis[-m-(thiocyanatomethyl)phenyl]porphyrin(Zn-28).

A mixture of 38 mg of10,20-diphenyl-5,15-bis[m-(thiocyanatomethyl)phenyl]porphyrin (28, 50.2μmol) in 30 mL of CH₂Cl₂ and a solution of 140 mg of Zn(OAc)₂.2H₂O (0.64mmol) in 5 mL of methanol were combined and stirred at ambienttemperature. After completion of the metalation (checked by fluorescenceexcitation spectroscopy) 20 mL of H₂O were added. The phases wereseparated and the organic layer was washed with 20 mL of 5% NaHCO₃ and20 mL of H₂O, dried (Na₂SO₄) and filtered. The solvents were removedunder reduced pressure. Column chromatography over flash silica gel withCH₂Cl₂/hexanes (5:1) gave 22 mg (26.8 μmol, 53% yield) of a dark purplesolid. IR (neat): {tilde over (v)}=3049 (w, arom. CH), 2924 (s, CH),2853 (m, CH), 2154 (m, CN), 1598 (m, arom. C═C), 1522 (w), 1480 (m),1440 (m), 1339 (m), 1206 (m), 1070 (m), 1002 (s), 934 (w), 796 (s), 741(m), 703 (s), 662 (m); ¹H NMR (300.5 MHz, CDCl₃): δ=4.44 (s, 4H, CH₂),7.70–7.90 (m, 11H, ArH), 8.16–8.28 (m, 8H, ArH), 8.93 (d, 4H, β-pyrrole,³J=4.2 Hz), 8.98 (d, 4H, β-pyrrole); LD-MS calcd av mass 820.31, obsd819.6, 792.3 [M⁺−CN], 761.5 [M⁺−SCN], 703.3 [M⁺−2 SCN], 626.0 [M⁺−2SCN—C₆H₅], 613.5 [M⁺−2 SCN—C₇H₇]; FAB-MS obsd 818.1275, calcd exact mass818.1265 (C₄₈H₃₀N₆S₂Zn); λ_(abs) (toluene) 424, 550, 590 nm; λ_(em)(toluene) 599, 647 nm.

5,10,15,20-Tetrakis[m-(S-acetylthiomethyl)phenyl]porphyrin (29).

A solution of 101 mg of5,10,15,20-tetrakis[m-(bromomethyl)phenyl]porphyrin (Wen et al. (1997)J. Am. Chem. Soc., 119: 7726–7733, Karaman et al. (1992) J. Am. Chem.Soc., 114: 4889–4898) (102 μmol) and 60 mg of potassium thioacetate (525μmol) in 20 mL of THF was refluxed. After 5 h the mixture was cooled toroom temperature. 30 mL of water was added. The mixture was cooled toroom temperature and the phases were separated. The organic phase waswashed with 40 mL of 5% NaHCO₃ solution and dried (Na₂SO₄). Columnchromatography over flash silica gel with THF afforded a purple wax,which was purified by refluxing in hexanes. The mixture was filtered andthe residue was dissolved in CH₂Cl₂. The solvent was removed underreduced pressure, affording 63 mg (65 μmol, 63% yield) of a purplesolid. IR (neat) {tilde over (v)}=3423 (m, NH), 3318 (m, NH), 2963 (w,CH), 2926 (w, CH), 1690 (s, CO), 1600 (w), 1562 (w), 1540 (w), 1508 (w),1472 (w), 1420 (w), 1351 (w), 1132 (m), 1103 (w), 1018 (w), 997 (w), 957(w), 917 (w), 800 (m), 718 (m); ¹H NMR (300.5 MHz, CDCl₃): δ=−2.83 (s,2H, NH), 2.40 (s, 12H, CH₃), 4.41 (s, 8H, CH₂), 7.65–7.75 (m, 8H, ArH),8.06–8.17 (m, 8H, ArH), 8.84 (s, 8H, β-pyrrole); LD-MS calcd av mass966.2 (C₅₆H₄₆N₄O₄S₄), obsd 967.4, 925.3 [M⁺−COCH₃], 892.2 [M⁺−SCOCH₃],850.0 [M⁺−SCOCH₃—COCH₃], 817.2 [M⁺−2 SCOCH₃], 775.5 [M⁺−2 SCOCH₃—COCH₃;λ_(abs) (toluene) 421, 515, 550, 591, 648 nm.

Zinc(II)-5,10,15,20-Tetrakis[m-(S-acetylthiomethyl)phenyl]porphyrin(Zn-29).

A mixture of 16.2 mg of 29 (16.7 μmol) in 20 mL of CHCl₃ and a solutionof 80.0 mg of Zn(OAc)₂.2H₂O (365 μmol) in 5 mL of methanol were combinedand stirred at ambient temperature. After 2 h the metalation wascompleted (checked by fluorescence excitation spectroscopy) and 40 mL ofH₂O was added. The phases were separated and the organic layer waswashed three times with 5% NaHCO₃ and dried (Na₂SO₄). The solvents wereremoved under reduced pressure. Column chromatography over flash silicagel with CH₂Cl₂/hexanes (4:1) gave the title compound as a purple solidin quantitative yield. IR (neat): {tilde over (v)}=2922 (w, CH), 2849(w, CH), 1690 (s, CO), 1655 (m, arom. C═C), 1600 (w, arom. C═C), 1478(w), 1420 (w), 1338 (w), 1208 (m), 1131 (m), 1067 (w), 1002 (m), 955(w), 932 (w), 794 (m), 717 (m); ¹H NMR (300.5 MHz, CDCl₃): δ=2.30 (s,12H, CH₃), 4.31 (s, 8H, CH₂), 7.62–7.69 (m, 8H, ArH), 8.05–8.13 (m, 8H,ArH), 8.92 (s, 8H, β-pyrrole); LD-MS calcd av mass 1028.15(C₅₆H₄₄N₄O₄S₄Zn), obsd 1028.8, 986.6 [M⁺−COCH₃], 954.7 [M⁺−SCOCH₃],911.3 [M⁺−SCOCH₃—COCH₃], 880.4 [M⁺−2 SCOCH₃], 838.8 [M⁺−2 SCOCH₃—COCH₃],805.3 [M⁺−3 SCOCH₃]; λ_(ab)'s(toluene) 424, 550, 589 nm; λ_(em)(toluene) 597, 647 nm.

Cobalt(II)-5,10,15,20-Tetrakis[m-(S-acetylthiomethyl)phenyl]porphyrin(Co-29).

A mixture of 14.2 mg of 29 (14.7 μmol) in 20 mL of CHCl₃ and a solutionof 60.0 mg of Co(OAc)₂.4H₂O (339 μmol) in 5 mL of methanol were combinedand stirred at ambient temperature. After 5 h an additional 261.0 mg ofCo(OAc)₂.4H₂O (1.5 mmol) was added because there was still free baseporphyrin left. Stirring at room temperature was continued. After 20 hthe metalation was completed (checked by fluorescence excitationspectroscopy) and 30 mL of H₂O was added. The phases were separated andthe organic layer was washed three times with 5% NaHCO₃ and dried(Na₂SO₄). The solvents were removed under reduced pressure. Columnchromatography over flash silica gel with CH₂Cl₂/hexanes (5:1) gave thetitle compound as an orange-purple solid in quantitative yield. IR(neat): {tilde over (v)}=3037 (w, arom. CH), 2955 (w, CH), 2924 (m, CH),2849 (w, CH), 1725 (w), 1693 (s, CO), 1601 (w, arom. C═C), 1455 (w),1422 (w), 1350 (m), 1131 (m), 1003 (m), 957 (w), 796 (m), 714 (m); ID-MScalcd av mass 1023.16 (C₅₆H₄₄N₄O₄S₄Co), obsd 1023.4, 980.3 [M⁺−COCH₃],948.3 [M⁺−SCOCH₃], 875.2 [M⁺−2 SCOCH₃]; λ_(abs) (toluene) 414, 529 nm.

Example 2 Setting and Reading the State of a Porphyrinic Macrocycle

I. Preparation of Gold Electrodes, Formation of Electrochemical Cell,Deposition of Thiol-porphyrin Monolayer.

Glass slides were soaked in 90° C. piranha solution for thirty minutes,thoroughly rinsed with doubly distilled water, and dried under vacuum. A1 nm layer of chromium was evaporated onto the glass, followed by 100 nmof gold through a thin mask consisting of four parallel lines, each witha width of approximately 75 microns, spaced at approximately 1 mmintervals. All depositions were done at 10⁻⁶ torr using an E-beamevaporator.

Immediately after venting of the vacuum system, the slides were removedand stored under dry ethanol until use. The slides were dried with astream of nitrogen and a piece of PDMS with a 3 mm diameter hole in thecenter was immediately placed over all four gold electrodes and filledwith a porphyrin solution (0.1 mg per milliliter in dry ethanol) (Z-15from example 1).

The slide was then sonicated at room temperature for 15 minutes whichwas found to facilitate monolayer formation. After sonication, the PDMSmask was removed and the slide was rinsed with dry ethanol. A new PDMSmask was prepared by casting a 10:1 ratio solution of monomer tocatalyst into a mold consisting of a pyramidal channel with a 40 μm by 1cm base width. This new mask was placed on top of the porphyrin-coveredelectrodes to form the electrochemical cell. The channel was filled with1.0 M TBAP, and a silver wire reference electrode was used to completethe electrical circuit. This creates four identical porphyrin-coveredgold electrodes with 40 by 75 micron dimensions, each of which isindividually addressable using a common backplane reference electrode.The porphyrin monolayer was then analyzed with cyclic voltammetry toestablish that the porphyrin had bound to the gold substrate and toestablish the extent of coverage of the monolayer on the gold surface(FIG. 23).

II. Reading and Writing Porphyrin Bits.

A labview program was written to apply a potential pulse (the pulse wasapplied to the reference electrode, since the working electrode wasmaintained at ground potential). Thus, the potential was inverted andapplied to the reference electrode. The waveform was generated at 5 MHzand applied to a bare silver wire reference electrode. The currentresponse was monitored through the gold working electrode. The referenceelectrode was poised at a constant DC potential using a home builtpotentiostat that also amplifies the resulting current.

In order to write a bit into the porphyrin monolayer, it was necessaryto apply the appropriate potential to create the appropriate oxidationstate of the porphyrin. The reference electrode was poised at three DCpotentials while the working electrode was held at zero potential, inorder to probe the response at the neutral and at both non-neutraloxidation states of the porphyrin (FIG. 23). A 0–300 mV potential pulsewas applied below the first oxidation potential to record the backgroundcharging current. At 300 mV, there was no redox process occurring, thus,only the background charging current was observed. The electrode wasthen set at 500 mV DC and an identical 300 mV potential pulse wasapplied raising the potential to 800 mV and thereby eliciting the firstoxidation of the porphyrin.

This current response was the sum of the faradaic current superimposedon the background charging current. Because the background was constant,the first response could be subtracted from the latter and the remainderwas the faradaic current.

A second potential step was applied from 800–1100 mV. This step oxidizedthe porphyrin into the second oxidation state and produced a secondincrement of faradaic current which again was background subtracted. Thebackground subtracted currents had approximately equal magnitude becauseeach corresponded to a one electron processes in the same molecule(s)immobilized to the electrode surface. The amplified signal was acquiredat 5 MHz giving a time resolution of 200 ns per data point which wassufficient to detect the roughly 70 μs transient response. Thebackground-subtracted instantaneous current was integrated to produce aplot of the instantaneous charge as a function of time (FIG. 23).

Once the porphyrin was set at a given oxidation state, it could be readby applying the appropriate negative potential step. For example, thehigher bit could be read simply by stepping between 1100–800 mV. Thelower bit could be read by stepping between 800–500 mV. The chargingcurrent could be determined by stepping between 300–0 mV. Again, thebackground was subtracted from each step to determine thebackground-subtracted read current (FIG. 25). The read/write cycles areillustrated in FIG. 24.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

1. An apparatus for storing data, said apparatus comprising: a fixed electrode electrically coupled to a storage medium having a multiplicity of different and distinguishable oxidation states wherein data is stored in said oxidation states by the addition or withdrawal of one or more electrons from said storage medium via the electrically coupled electrode.
 2. The apparatus of claim 1, wherein said storage medium stores data at a density of at least one bit per molecule.
 3. The apparatus of claim 1, wherein said storage medium comprises a molecule having at least two different and distinguishable oxidation states.
 4. The apparatus of claim 1, wherein said storage medium is covalently linked to said electrode.
 5. The apparatus of claim 1, wherein said storage medium is electrically coupled to said electrode through a linker.
 6. The apparatus of claim 1, wherein said storage medium is covalently linked to said electrode through a linker.
 7. The apparatus of claim 6, wherein said linker is a thiol linker.
 8. The apparatus of claim 1, wherein said storage medium is juxtaposed in the proximity of said electrode such that electrons pass from said storage medium to said electrode.
 9. The apparatus of claim 1, wherein said storage medium is juxtaposed to a dielectric material imbedded with counterions.
 10. The apparatus of claim 1, wherein said storage medium and said electrode are fully encapsulated in an integrated circuit.
 11. The apparatus of claim 1, wherein said storage medium is electronically coupled to a second fixed electrode that is a reference electrode.
 12. The apparatus of claim 1, wherein said storage medium is present on a single plane in said apparatus.
 13. The apparatus of claim 1, wherein said storage medium is present at a multiplicity of storage locations.
 14. The apparatus of claim 13, wherein said storage locations are present on a single plane in said apparatus.
 15. The apparatus of claim 13, wherein said apparatus comprises multiple planes and said storage locations are present on multiple planes of said apparatus.
 16. The apparatus of claim 13, wherein said storage locations range from about 1024 to about 4096 different locations.
 17. The apparatus of claim 16, wherein each location is addressed by a single electrode.
 18. The apparatus of claim 16, wherein each location is addressed by two electrodes.
 19. The apparatus of claim 1, wherein said electrode is connected to a voltage source.
 20. The apparatus of claim 19, wherein said voltage source is the output of an integrated circuit.
 21. The apparatus of claim 1, wherein said electrode is connected to a device to read the oxidation state of said storage medium.
 22. The apparatus of claim 21, wherein said device is selected from the group consisting of a voltammetric device, an amperometric device, and a potentiometric device.
 23. The apparatus of claim 22, wherein said device is an impedance spectrometer or a sinusoidal voltammeter.
 24. The apparatus of claim 21, wherein said device provides a Fourier transform of the output signal from said electrode.
 25. The apparatus of claim 21, wherein said device refreshes the oxidation state of said storage medium after reading said oxidation state.
 26. The apparatus of claim 1, wherein said different and distinguishable oxidation states of said storage medium can be set by a voltage difference no greater than about 2 volts.
 27. The apparatus of claim 26, wherein said storage medium comprises a porphyrinic macrocycle substituted at a β-position or at a meso-position.
 28. The apparatus of claim 1, wherein said storage medium comprises a molecule having the formula:

wherein M¹ and M² are independently selected metals; S¹, S², S³, S⁴, S⁵, S⁶, S⁷, and S⁸ are independently selected from the group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are independently selected from the group consisting of are independently selected from the group consisting of N, O, S, Se, Te, and CH; L¹ and L² are independently selected linkers; and X¹ and X² are independently selected from the group consisting of a substrate, a reactive site that can covalently couple to a substrate, and a reactive site that can ionically couple to a substrate.
 29. The apparatus of claim 28, wherein S¹, S², S³, S⁵, S⁶, and S⁷ are the same; S⁴ and S⁸ are the same; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are the same J¹, J², J³ and J⁴ are the same; and M¹ and M² are different.
 30. The apparatus of claim 29, wherein said storage medium comprises a molecule having the formula:

wherein X¹ and X² are independently selected from the group consisting of H and a substrate.
 31. A method of storing data, said method comprising: i) providing an apparatus according to claim 1; and ii) applying a voltage to said electrode at sufficient current to set an oxidation state of said storage medium.
 32. The method of claim 31, wherein said voltage ranges up to about 2 volts.
 33. The method of claim 31, wherein said voltage is the output of an integrated circuit.
 34. The method of claim 31, wherein said voltage is the output of a logic gate.
 35. The method of claim 31, further comprising detecting the oxidation state of said storage medium and thereby reading out the data stored therein.
 36. The method of claim 35, wherein said detecting the oxidation state of the storage medium further comprises refreshing the oxidation state of the storage medium.
 37. The method of claim 35, wherein said detecting comprises analyzing a readout signal in the time domain.
 38. The method of claim 35, wherein said detecting comprises analyzing a readout signal in the frequency domain.
 39. The method of claim 38, wherein said detecting comprises performing a Fourier transform on said readout signal.
 40. The method of claim 35, wherein said detecting utilizes a voltammetric method.
 41. The method of claim 35, wherein said detecting utilizes impedance spectroscopy.
 42. The method of claim 35, wherein said detecting comprises exposing said storage medium to an electric field to produce an electric field oscillation having characteristic frequency and detecting said characteristic frequency.
 43. The method of claim 31, wherein said storage medium comprises a molecule selected from the group consisting of a porphyrinic macrocycle, a metallocene, a linear polyene, a cyclic polyene, a heteroatom-substituted linear polyene, a heteroatom-substituted cyclic polyene, a tetrathiafulvalene, a tetraselenafulvalene, a metal coordination complex, a buckyball, a triarylamine, a 1,4-phenylenediamine, a xanthene, a flavin, a phenazine, a phenothiazine, an acridine, a quinoline, a 2,2′-bipyridyl, a 4,4′-bipyridyl, a tetrathiotetracene, and a peri-bridged naphthalene dichalcogenide.
 44. The method of claim 43, wherein said storage medium comprises a molecule selected from the group consisting of a porphyrin, an expanded porphyrin, a contracted porphyrin, a ferrocene, a linear porphyrin polymer, and a porphyrin array.
 45. The method of claim 43, wherein said storage medium comprises a porphyrinic macrocycle substituted at a β-position or at a meso-position.
 46. The method of claim 43, wherein said molecule has at least eight different and distinguishable oxidation states.
 47. In a computer system, a memory device, said memory device comprising the apparatus of claim
 1. 48. A computer system comprising a central processing unit, a display, a selector device, and a memory device, said memory device comprising the apparatus of claim
 1. 49. An information storage medium, said storage medium comprising a one or more storage molecules such that said storage medium has at least two different and distinguishable non-neutral oxidation states.
 50. The storage medium of claim 49, wherein said storage molecule has the formula:

wherein M¹ and M² are independently selected metals; S¹, S², S³, S⁴, S⁵, S⁶, S⁷, and S⁸ are independently selected from the group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are independently selected from the group consisting of are independently selected from the group consisting of N, O, S, Se, Te, and CH; L¹ and L² are independently selected linkers; and X¹ and X² are independently selected from the group consisting of a substrate, a reactive site that can covalently couple to a substrate, and a reactive site that can ionically couple to a substrate.
 51. The storage medium of claim 50, wherein S¹, S², S³, S⁵, S⁶, and S⁷ are the same; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are the same J¹, J², J⁴ and J⁵ are the same; and M¹ and M² are different.
 52. The storage medium of claim 51, wherein said storage molecule has the formula:

wherein X¹ and X² are independently selected from the group consisting of H and a substrate.
 53. The storage medium of claim 49, wherein each storage molecule is present at a discrete storage location on a substrate.
 54. The storage medium of claim 49, wherein the storage molecule is in contact with a dielectric material imbedded with counterions.
 55. The storage medium of claim 49, wherein said storage molecule comprises two or more covalently linked redox-active subunits.
 56. A molecule for the storage of information, said molecule having the formula:

wherein M¹ and M² are independently selected metals; S¹, S², S³, S⁴, S⁵, S⁶, S⁷, and S⁸ are independently selected from the group consisting of aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; K¹, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are independently selected from the group consisting of are independently selected from the group consisting of N, O, S, Se, Te, and CH; L¹ and L² are independently selected linkers; and X¹ and X² are independently selected from the group consisting of a substrate, a reactive site that can covalently couple to a substrate, and a reactive site that can ionically couple to a substrate. 